<PAGE>   1


                                  UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                                   FORM 20-F/A

(Mark One)

          [ ]  REGISTRATION STATEMENT PURSUANT TO SECTION 12(b) OR (g) OF THE
               SECURITIES EXCHANGE ACT OF 1934

                                       OR

          [X]  ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES
               EXCHANGE ACT OF 1934

For the fiscal year ended December 31, 1999

                                       OR

          [ ]  TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
               SECURITIES EXCHANGE ACT OF 1934

For the transition period from ______________________ to __________________

Commission file number               0-29574
                       ------------------------------------------------------

                                  ALTAREX CORP.
             (Exact name of Registrant as specified in its charter)

                           Province of Alberta, Canada
                 (Jurisdiction of incorporation or organization)

                       303 Wyman Street, Waltham, MA 02451
                    (Address of principal executive offices)

Securities registered or to be registered pursuant to Section 12(b) of the Act:

Title of each Class                    Name of each exchange on which registered

     None
--------------------------------------------------------------------------------

Securities registered or to be registered pursuant to Section 12(g) of the Act:


<PAGE>   2


                         Common Shares without par value
                                (Title of Class)

Securities for which there is a reporting obligation pursuant to Section 15(d)
of the Act:

                                      None
                                (Title of Class)

Number of outstanding shares of each of the Company's classes of capital or
common stock as of September 15, 2000: 74,037,958 Common Shares

Indicate by check mark whether the registrant (1) has filed all reports required
to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during
the preceding 12 months (or for such shorter period that the registrant was
required to file such reports), and (2) has been subject to such filing
requirements for the past 90 days.

Yes  X     No ---
    ---
Indicate by check mark which financial statement item the registrant has elected
to follow.

Item 17  X     Item 18 ----
        ---
(APPLICABLE ONLY TO ISSUERS INVOLVED IN BANKRUPTCY PROCEEDINGS DURING THE PAST
FIVE YEARS)

Indicate by check mark whether the registrant has filed all documents and
reports required to be filed by Sections 12, 13 or 15(d) of the Securities
Exchange Act of 1934 subsequent to the distribution of securities under a plan
confirmed by a court.

Yes ___   No ___


<PAGE>   3
     AltaRex Corp. (the "Company") hereby amends its Annual Report on Form 20-F
for the year ended December 31, 1999 to include the exhibits filed with and
attached to this Amendment.
<PAGE>   4


                                    SIGNATURE

Pursuant to the requirements of Section 12 of the Securities Exchange Act of
1934, the registrant certifies that it meets all of the requirements for filing
on Form 20-F and has duly caused this registration statement to be signed on its
behalf by the undersigned, thereunto duly authorized.

ALTAREX CORP.


Per:  /s/ Edward M. Fitzgerald
     --------------------------------------
         Edward M. Fitzgerald
         Senior Vice President and
         Chief Financial Officer

DATED this 21st day of September, 2000.


                                      3
<PAGE>   5

<TABLE>
<CAPTION>
                                  EXHIBIT INDEX

EXHIBIT                                                                                                     PAGE
NUMBER                                             DOCUMENT DESCRIPTION                                    NUMBER
------                                             --------------------                                    ------

<S>                 <C>                                                                                    <C>
1.1+                The Articles of the Company dated November 18, 1993 as amended by Articles of
                    Amendment dated June 25, 1996 and November 28, 1996.
1.2+                Articles of Amalgamation of the Company dated May 31, 1997 as amended by Articles
                    of Amendment dated June 27, 1997.
1.3+                The Bylaws of the Company dated March 1, 1995.
1.4+                Asset Purchase Agreement dated November 24, 1995 among AltaRex Inc., Biomira
                    Research Inc. and Biomira Inc.*
1.5+                The Share Purchase Agreement.
1.6**               Settlement Agreement by and among Biomira Inc. and AltaRex Corp., dated September 3,
                    1999.
1.7+                Letter Agreement dated February 20, 1996 between AltaRex Corp. and Merck Frosst
                    Canada Inc.
1.8**#              Amended and Restated License Agreement between Biomira Inc. and AltaRex Corp., dated
                    September 3, 1999.
1.9**#              Patent License Agreement between AltaRex Corp. and the National Institute of Health, the
                    Centers for Disease Control and Prevention and the Food and Drug Administration, dated
                    September 21, 1999.
1.10+               License Agreement dated November 24, 1995 between Biomira Inc. and AltaRex Corp.*
1.11+               Assignment of Patent Agreement dated April 4, 1996 between Biomira Inc. and
                    AltaRex Corp.
1.12+               Employment Contracts dated January 1, 1996, between AltaRex Corp. and Dr. Antoine Noujaim.
1.13**              Stock Option Plan
1.15+               Assignment of Letter Agreement dated February 20, 1996 between AltaRex Corp. and Merck Frosst
                    Canada Inc. See "Business of AltaRex -- Strategic Alliances and License Agreement."
1.16+               Sublease Agreement by and between Tufts Associated Health Plans, Inc. and AltaRex U.S. Corp.
                    dated April 15, 2000.
1.17+               Employment Arrangement dated February 18, 1998 and amended as of March 30, 1998 between
                    AltaRex Corp. and Richard E. Bagley.
1.18+               Amendments to Employment Arrangement between AltaRex Corp. and Richard E. Bagley,
                    dated June 1, 1999 and December 22, 1999.
1.19+               Amendments to Employment Contract between AltaRex Corp. and Dr. Antoine Noujaim
                    dated June 3, 1999 and December 22, 1999.
1.20+               Employment Contract between AltaRex Corp. and Dr. Christopher Nicodemus dated December 16, 1998,
                    as amended on June 1, 1999 and December 22, 1999.
1.21+               Employment Contract between AltaRex Corp. and Edward Fitzgerald dated September 14, 1998,
                    as amended on June 1, 1999 and December 22, 1999.
1.22+               Employment Contract between AltaRex Corp. and Peter C. Gonze dated January 24,
                    2000.
</TABLE>

*    Confidential treatment granted as to certain portions, which portions are
     omitted and filed separately with the Commission.

**   Filed herewith.

+    Incorporated by reference to Exhibits to the Company's Registration
     Statement on Form 20-F.

#    Confidential treatment has been requested for certain portions of this
     Exhibit. The confidential redacted information has been filed separately
     with the Securities and Exchange Commission.