QuickLinks -- Click here to rapidly navigate through this document

---

---

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington D.C. 20549

---

FORM 11-K

ý	ANNUAL REPORT PURSUANT TO SECTION 15(D) OF THE SECURITIES EXCHANGE ACT OF 1934
	FOR THE PLAN YEAR ENDED DECEMBER 31, 2003
or
o	TRANSITION REPORT PURSUANT TO SECTION 15(D) OF THE SECURITIES EXCHANGE ACT OF 1934
Commission File No.: 1-31349
A.	Full title of the plan and address of the plan, if different from that of the issuer named below:
THE THOMSON 401(k) SAVINGS PLAN
B.	Name of issuer of the securities held pursuant to the plan and the address of its principal executive office:
THE THOMSON CORPORATION
Metro Center, One Station Place Stamford, Connecticut 06902

---

---

---

THE THOMSON 401(K) SAVINGS PLAN
INDEX TO FINANCIAL STATEMENTS AND EXHIBITS

			Page
Report of Independent Registered Public Accounting Firm			1
Financial Statements:
	Statements of Net Assets Available for Benefits as of December 31, 2003 and 2002		2
	Statement of Changes in Net Assets Available for Benefits for the Year Ended December 31, 2003		3
	Notes to Financial Statements		4-11
Supplemental Schedule: *
	Form 5500, Part IV of Schedule H, Line 4i, Schedule of Assets (Held at End of Year) — as of December 31, 2003		12
Signature			13
Exhibits:
	Consent of Independent Registered Public Accounting Firm

* Other schedules required by Section 2520.103-10 of the Department of Labor's Rules and Regulations for Reporting and Disclosure under the Employee Retirement Income Security Act of 1974, as amended, have been omitted because they are not applicable.

---

Report of Independent Registered Public Accounting Firm

To the Participants and Administrator of
The Thomson 401(k) Savings Plan

        In our opinion, the accompanying statements of net assets available for benefits and the related statement of changes in net assets available for benefits present fairly, in all material respects, the net assets available for benefits of The Thomson 401(k) Savings Plan (the "Plan") at December 31, 2003 and 2002, and the changes in net assets available for benefits for the year ended December 31, 2003 in conformity with accounting principles generally accepted in the United States of America. These financial statements are the responsibility of the Plan's management. Our responsibility is to express an opinion on these financial statements based on our audits. We conducted our audits of these statements in accordance with the standards of the Public Company Accounting Oversight Board (United States). Those standards require that we plan and perform the audit to obtain reasonable assurance about whether the financial statements are free of material misstatement. An audit includes examining, on a test basis, evidence supporting the amounts and disclosures in the financial statements, assessing the accounting principles used and significant estimates made by management, and evaluating the overall financial statement presentation. We believe that our audits provide a reasonable basis for our opinion.

        Our audits were conducted for the purpose of forming an opinion on the basic financial statements taken as a whole. The supplemental schedule of Assets Held at End of Year is presented for the purpose of additional analysis and is not a required part of the basic financial statements but is supplementary information required by the Department of Labor's Rules and Regulations for Reporting and Disclosure under the Employee Retirement Income Security Act of 1974. This supplemental schedule is the responsibility of the Plan's management. The supplemental schedule has been subjected to the auditing procedures applied in the audits of the basic financial statements and, in our opinion, is fairly stated in all material respects in relation to the basic financial statements taken as a whole.

/s/ PricewaterhouseCoopers LLP

New York, NY
June 24, 2004

1

---

The Thomson 401(k) Savings Plan

Statements of Net Assets Available for Benefits
December 31, 2003 and 2002

				2003			2002
Assets
Investments, at fair value or contract value (Note 4)				$	1,441,925,022		$	1,121,274,632
Cash					59,571			845
Participant loans					31,546,061			29,365,582
Contributions receivable
	Employer				4,186,332			5,967,581
	Employee				828			499,700
		Total assets			1,477,717,814			1,157,108,340
Liabilities
Accrued expenses					113,695			—
		Total liabilities			113,695			—
		Net assets available for benefits		$	1,477,604,119		$	1,157,108,340

The accompanying notes are an integral part of these financial statements.

2

---

The Thomson 401(k) Savings Plan

Statement of Changes in Net Assets Available for Benefits
Year Ended December 31, 2003

				2003
Additions to net assets
Investment income
	Interest and dividends			$	22,014,785
	Net appreciation in fair value of investments (Note 4)				234,758,005
		Total investment income			256,772,790
Contributions
	Employee				115,036,288
	Employer				46,140,183
	Rollovers				6,080,946
		Total contributions			167,257,417
		Increase to net assets			424,030,207
Deductions from net assets
Benefits paid to participants					102,301,207
Forfeitures utilized for Plan administrative expenses (Note 1)					505,293
Loan origination fees					119,407
		Total deductions from net assets			102,925,907
		Net increase in net assets available for benefits before other transfers			321,104,300
Other transfers out (Note 1)					(608,521	)
		Net increase in net assets available for benefits after other transfers			320,495,779
Net assets available for benefits
Beginning of year					1,157,108,340
End of year				$	1,477,604,119

The accompanying notes are an integral part of these financial statements.

3

---

The Thomson 401(k) Savings Plan
Notes to Financial Statements
December 31, 2003

1.

Description of the Plan

The following description of The Thomson 401(k) Savings Plan (the "Plan") is provided for general information purposes only. Participants should refer to the Plan agreement for more complete information.

General

The Plan is a defined contribution plan covering substantially all of the employees of Thomson Holdings Inc. and certain affiliates (the "Company"). The Plan is subject to the provisions of the Employee Retirement Income Security Act of 1974 ("ERISA").

Participation

All full-time employees of the Company are eligible to participate, provided that the employee has completed at least 90 days of service. Part-time employees are eligible to participate upon completion of one year of employment.

Investments

During 2003, the Plan began offering units representing shares of common stock of The Thomson Corporation as an investment option for participants.

Contributions

In accordance with the provisions of the Plan, participants may voluntarily authorize the Company to contribute on a pretax, or contribute on a post-tax basis, ¹/2% to 16% of their annual base salary and overtime through Salary Reduction (as defined in the Plan agreement) contributions for investment in the Plan, up to the maximum allowed under the provisions of the Internal Revenue Code ("IRC"). The Company will make matching contributions of fifty cents on each dollar the participant contributes, up to 6% of the participant's compensation. The Company honors other contribution provisions for plans that were merged into the Plan, up to a 100% match of 6% of the participant's compensation. Additionally, for any Plan year, the Company may make discretionary contributions to the Plan.

Vesting of benefits

A participant is fully vested in his or her contributions and the earnings thereon at all times and will become vested in Company contributions and the related earnings credited to their account, based upon their years of service, as shown below:

Employees who reach normal retirement age, or become disabled or die while still employed by the Company, are immediately vested in the value of the Company contributions credited to their accounts.

4

---

Distributions and forfeitures

Distributions from participant accounts under the Plan as a result of termination, retirement, disability or death will be paid at the election of the participant (beneficiary) as either: (a) a lump-sum (required if the total benefit is less than $5,000); (b) substantially equal annual or more frequent installments over a reasonable period of time not to exceed the participant's life expectancy or the joint and survivor life expectancy of the participant and their designated beneficiary at the commencement of the installments; or (c) a combination of the foregoing.

Participant balances which have not been vested will generally be forfeited upon termination of employment. At December 31, 2003 and 2002, forfeited nonvested accounts, which remain as assets in the Plan, totaled $390,716 and $437,969, respectively. Amounts forfeited by withdrawing members may be applied against future matching employer contributions or employer discretionary contributions or used to pay Plan administrative expenses. In 2003, employer contributions were reduced by $1,280,886 from forfeited nonvested accounts. Forfeitures that have been used to pay Plan administrative expenses have been reflected as such in the accompanying financial statements.

Loans to participants

Participants may borrow from their accounts for any purpose. The minimum loan amount is $500 and the maximum is 50% of the participant's vested balance up to the Internal Revenue Service limit of $50,000. Participants may have up to two outstanding loans at a time. Additionally, loans must be repaid by payroll deductions over a period not to exceed five years, except when the loan is used for the purchase of a primary residence, in which case the repayment period may not exceed ten years. Some loans transferred into the Plan as a result of Plan mergers have repayment periods up to 30 years. Interest rates on loans are determined based upon the prevailing interest rate charged for similar loans by commercial banks in the area of the employer's place of business.

Plan administration

A Plan administrator, appointed by the board of directors of the Company, administers the Plan in accordance with the terms and provisions of the Plan agreement. The Plan administrator has appointed Putnam Fiduciary Trust Company ("Putnam") as the trustee and third party administrator to manage the assets and day-to-day operations of the Plan.

Termination

The Company anticipates and believes that the Plan will continue without interruption but reserves the right to terminate, amend or modify the Plan at any time subject to the provisions of ERISA. Upon termination of the Plan, the balances in the accounts of each participant would become fully vested and be distributed to participants as described above.

5

---

Transfers

During 2003, Plan assets totaling $608,521 were transferred from the Plan either into a separate plan maintained by the Company or as a result of the sale of certain business units of the Company as follows:

2.

Summary of Significant Accounting Policies

Basis of accounting

The accompanying financial statements have been prepared on the accrual basis of accounting.

Investment valuation and transactions

Investments (excluding the commingled trusts and the Putnam Stable Value Fund) are valued at fair market value based on quoted market prices. The commingled trusts are valued at fair market value based on quoted market prices of the underlying securities within that fund. The Putnam Stable Value Fund is recorded at contract value. Participant loans are valued at cost less principal repayments, which approximates fair value. Purchases and sales of shares in the investment funds are recorded on the trade date. Interest income is recorded on the accrual basis. Dividends are recorded on the ex-dividend date. The Plan presents, in the statement of changes in net assets available for benefits, the net appreciation (depreciation) in the fair value of its investments which consists of the realized gains or losses and the unrealized appreciation (depreciation) on those investments.

Administrative expenses

Administrative expenses of the Plan are either paid directly by the Company or are funded utilizing Plan forfeitures. Administrative expenses paid by the Company are not reflected in the accompanying financial statements. Administrative expenses paid out of the forfeitures account have been reflected in the accompanying financial statements. Loan origination fees are paid by the participant via a deduction from their respective account, and are recorded upon issuance of the loan.

Benefit payments

Benefits are recorded when paid.

Use of estimates

The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the reported amounts of net assets available for benefits and changes therein, and the disclosure of contingent assets and liabilities. Actual results could differ from those estimates.

6

---

Risks and uncertainties

The Plan provides for various investment options in mutual funds that invest in any combination of stocks, bonds, fixed income securities and other investment securities. These investment securities are exposed to various risks, such as interest rate, market and credit risks. Due to the level of risk and uncertainty associated with certain investment securities, it is at least reasonably possible that changes in the values of investment securities will occur in the near term and that such changes could materially affect participants' account balances and the amounts reported in the statements of net assets available for benefits, and the statement of changes in net assets available for benefits.

Reclassifications

Certain reclassifications have been made to conform with current year presentation.

3.

Description of Investment Options

Participants in the Plan may elect to invest in any of the following investment options:

The George Putnam Fund of Boston

The fund invests primarily in stocks as well as corporate and U.S. government bonds.

Putnam Fund for Growth & Income

The fund invests in a diversified portfolio of stocks, targeting stocks of mature companies that offer long-term growth potential while also providing income.

Putnam Voyager Fund

The fund invests primarily in the stocks of large and midsize companies expected to grow over time.

Putnam New Opportunities Fund

The fund invests primarily in the common stocks of companies with long-term growth potential.

Putnam Stable Value Fund

The fund primarily invests in investment contracts or similar investments issued by insurance companies, banks, and similar financial institutions.

Artisan Mid Cap Fund

The fund primarily invests in stocks of mid size companies that exhibit franchise characteristics.

PIMCO Total Return Fund II

The fund invests in a diversified portfolio of fixed-income securities with an average duration that varies between three and six years.

Templeton Institutional Funds, Inc. Foreign Equity Series

The fund primarily invests in the equity securities of companies located outside the U.S. including emerging markets.

Frank Russell Small Cap Fund

The fund invests primarily in the small-capitalization stocks of the U.S. stock market.

7

---

T. Rowe Price Value Fund

The fund invests primarily in common stocks believed to be undervalued. Income is a secondary objective.

BGI Equity Index Fund

The fund invests in the same stocks held in the S&P 500 Index.

BGI U.S. Debt Index Fund

The fund invests in a diversified portfolio of debt securities including U.S. Government bonds, corporate bonds, residential and commercial mortgage-backed securities, and asset-backed securities. The fund tracks the Lehman Brothers Aggregate Bond Index.

BGI Extended Equity Market Fund

The fund invests in the stocks of small and medium-sized U.S. companies.

BGI Russell 1000 Growth Fund

This fund invests in issues of large U.S. companies that have high price-to-book ratios and higher growth rates.

Thomson Stock Fund

The fund invests in the common stock of The Thomson Corporation.

4.     Investments

Investments held by the Plan at December 31, 2003 and 2002 are as follows:

8

---

The following represents investments that represent 5% or more of the Plan's net assets:

* The investment value of the fund in the year shown did not exceed 5% of the Plan's net assets.

Investment income (loss) of the Plan for the year ended December 31, 2003 is as follows:

During 2003, the Plan's investments (including gains and losses on investments bought and sold, as well as held during the year) appreciated in value by $234,758,005 as follows:

5.

Income Tax Status

The Internal Revenue Service has determined and informed management by a letter dated September 26, 2002, that the Plan is designed in accordance with the applicable sections of the Internal Revenue Code ("IRC"). Although the Plan has been amended subsequent to the receipt of the latest determination letter, the Plan administrator and the Plan's tax counsel believe that the Plan is designed and is currently being operated in compliance with the applicable requirements of the IRC. Accordingly, no provision for income taxes has been made in the accompanying financial statements.

9

---

6.

Reconciliation of Financial Statements to Form 5500

The following is a reconciliation of net assets available for benefits per the financial statements to the Form 5500 at:

The following is a reconciliation of benefits paid to participants per the financial statements for the year ended December 31, 2003 to the Form 5500:

Amounts allocated to withdrawing participants are recorded on the Form 5500 for benefit claims that have been processed and approved for payment prior to December 31, 2003, but not yet paid as of that date.

7.

Related Party Transactions

Certain Plan investments are mutual funds managed by Putnam. Putnam is the trustee as defined by the Plan, therefore, these transactions qualify as parties-in-interest.

Loan origination fees paid by the participants of the Plan amounted to $119,407 for the year ended December 31, 2003.

As of December 31, 2003, the Plan held 203,230 shares of The Thomson Corporation common stock valued at $7,367,081.

The employees of Jane's Information Group ("Jane's"), a business unit sold in April 2001 by the Company to its principal shareholder, The Woodbridge Company Limited, continue to participate in the Plan. Jane's matches the contributions of its employees in accordance with the provisions of the Plan.

10

---

8.

Subsequent Events

On April 1, 2004, assets totaling $19,460,036 related to the employees of the Drake Beam Morin business unit were transferred from the Plan as a result of the sale of that business unit on February 18, 2004.

Effective May 3, 2004, the Plan ceased to offer the Putnam New Opportunities Fund as an investment option. All existing asset balances were transferred to the Artisan Mid Cap Fund.

11

---

The Thomson 401(k) Savings Plan
Line 4i of Part IV of Schedule H
Schedule of Assets (Held at End of Year)
December 31, 2003

Schedule I

Identity of issuer			Description of investment		Fair value
Putnam Voyager Fund*			Open-end mutual fund		$	315,039,990
Putnam Stable Value Fund*			Collective investment trust			218,601,056	**
Putnam Fund for Growth & Income*			Open-end mutual fund			161,461,331
Putnam New Opportunities Fund*			Open-end mutual fund			145,109,922
BGI Equity Index Fund			Commingled trust			112,164,732
Templeton Institutional Funds, Inc.			Open-end mutual fund			92,232,547
The George Putnam Fund of Boston*			Open-end mutual fund			85,141,405
T. Rowe Price Value Fund			Open-end mutual fund			77,075,161
PIMCO Total Return Fund II			Open-end mutual fund			69,976,082
Frank Russell Small Cap Fund			Commingled trust			55,089,660
Artisan Mid Cap Fund			Open-end mutual fund			36,765,571
BGI Extended Equity Market Fund			Commingled trust			34,880,682
BGI U.S. Debt Index Fund			Commingled trust			28,632,959
Thomson Stock Fund*			Common stock fund			7,367,081
BGI Russell 1000 Growth Fund			Commingled trust			2,386,843
Cash						59,571
						1,441,984,593
Loan Fund*			Loans to participants (maturities range from 2003 to 2031; interest rates range from 5% to 11%)			31,546,061
	Total assets held for investment purposes				$	1,473,530,654

*

Parties-in-interest.

**

Valued at contract value.

12

---

SIGNATURE

        Pursuant to the requirements of the Securities Exchange Act of 1934, the Plan Administrator has duly caused this annual report to be signed on its behalf by the undersigned, thereunto duly authorized.

		THE THOMSON 401(K) SAVINGS PLAN
Date: June 28, 2004
		By:	/s/  JOHN J. RAFFAELI, JR.       Name: John J. Raffaeli, Jr. Title: Senior Vice President, Human Resources,          The Thomson Corporation

13

---

EXHIBIT INDEX

Exhibit Number		Description
23		Consent of Independent Registered Public Accounting Firm

14

---

QuickLinks

THE THOMSON 401(K) SAVINGS PLAN INDEX TO FINANCIAL STATEMENTS AND EXHIBITS
Report of Independent Registered Public Accounting Firm
SIGNATURE
EXHIBIT INDEX

QuickLinks -- Click here to rapidly navigate through this document

Exhibit 23

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

We hereby consent to the incorporation by reference in the Registration Statement on Form S-8 (No. 333-105280) of The Thomson Corporation of our report dated June 24, 2004 relating to the financial statements of The Thomson 401(k) Savings Plan, which appears in this Form 11-K.

/s/ PricewaterhouseCoopers LLP

New York, NY
June 24, 2004

---

QuickLinks

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM