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

                                   FORM 12b-25

                                                Commission File Number 000-19311


                           NOTIFICATION OF LATE FILING


         (Check One):  [X]  Form 10-K      [ ]  Form 20-F      [ ]  Form 11- K
                       [ ]  Form 10-Q      [ ]  Form N-SAR     [ ]  Form N-CSR

         For Period Ended: December 31, 2004

[ ]      Transition Report on Form 10-K   [ ]    Transition Report on Form 10-Q
[ ]      Transition Report on Form 20-F   [ ]    Transition Report on Form N-SAR
[ ]      Transition Report on Form 11-K

         For the Transition Period Ended:

         Read attached Instruction (on back page) before preparing form. Please
print or type.

         Nothing in this form shall be construed to imply that the Commission
has verified any information contained herein.

         If the notification relates to a portion of the filing checked above,
identify the item(s) to which the notification relates:


                        PART I -- REGISTRANT INFORMATION

Full name of registrant

                  Biogen Idec Inc.

Former name if applicable

                  IDEC Pharmaceuticals Corporation

Address of principal executive office (street and number)

                  14 Cambridge Center



City, state and zip code

                  Cambridge, Massachusetts 02142


                       PART II -- RULES 12b-25(b) AND (c)

         If the subject report could not be filed without unreasonable effort or
expense and the registrant seeks relief pursuant to Rule 12b-25(b), the
following should be completed. (Check box if appropriate.)

                (a)  The reasons described in reasonable detail in Part III of
                     this form could not be eliminated without unreasonable
                     effort or expense;

                (b)  The subject annual report, semi-annual report, transition
                     report on Form 10-K, Form 20-F, Form 11-K, Form N-SAR or
                     Form N-CSR, or portion thereof, will be filed on or before
                     the fifteenth calendar day following the prescribed due
     [X]             due date; or the subject quarterly report or transition
                     report on Form 10-Q, or portion thereof, will be filed on
                     or before the fifth calendar day following the prescribed
                     due date; and

                (c)  The accountant's statement or other exhibit required by
                     Rule 12b-25(c) has been attached if applicable.


                              PART III - NARRATIVE

         State below in reasonable detail the reasons why Forms 10-K, 20-F,
11-K, 10-Q, N-SAR, N-CSR, or the transition report or portion thereof, could not
be filed within the prescribed time period.

                  For the reasons described below, which could not be eliminated
         without unreasonable effort or expense, Biogen Idec Inc., or the
         Company, could not file its annual report on Form 10-K for the fiscal
         year ended December 31, 2004 within the prescribed 75-day time period.

                  As previously disclosed in a press release issued on February
         28, 2005 (furnished as Exhibit 99.1 of the Company's Current Report on
         Form 8-K furnished on March 3, 2005), the Company and Elan Corporation,
         plc, or Elan, in consultation with the U.S. Food and Drug
         Administration, voluntary suspended the marketing and commercial
         distribution of TYSABRI(R) (natalizumab), a treatment for multiple
         sclerosis, or MS, on February 28, 2005. In addition, the Company and
         Elan suspended dosing of TYSABRI in all clinical trials, including
         clinical trials in MS, Crohn's disease and rheumatoid arthritis. These
         decisions were based on reports of two serious adverse events that had
         occurred in patients treated with TYSABRI in combination with the
         Company's drug AVONEX(R) (interferon beta-1a) 


         in clinical studies. These events involved two cases, one confirmed at
         the time of the decision and one suspected and subsequently confirmed,
         of progressive multifocal leukoencephalopathy, or PML. The Company and
         Elan are working with clinical investigators to evaluate patients
         treated with TYSABRI in clinical trials and will consult with leading
         experts to better understand the possible risk of PML. Based on the
         full results of these evaluations, the Company and Elan, in
         consultation with regulatory authorities, will determine the
         appropriate next steps.

                  Since the February 28, 2005 announcement, the Company has been
         diligently assessing the impact of the suspension of TYSABRI, a
         subsequent event for purposes of its 2004 financial statements, and
         related matters on the Company's business, plans for the future and
         2004 financial statements, including reviewing certain of the Company's
         assets for possible impairment as of December 31, 2004. However, due to
         the recent occurrence of the subsequent event and its significance to
         the Company's Form 10-K disclosure, the Company has not been able to
         complete by March 16, 2005, without unreasonable effort or expense, its
         Form 10-K, including its 2004 financial statements and management's
         assessment of internal control over financial reporting as of December
         31, 2004. Management of the Company has not identified any material
         weakness (as defined in Public Company Accounting Oversight Board
         Auditing Standard No. 2) in the Company's internal control over
         financial reporting as of the date of this filing. 

                  The Company is proceeding expeditiously to complete this work
         and finalize the Form 10-K, and expects to file the Form 10-K within
         the time period specified in Rule 12b-25(b)(2)(ii).


                          PART IV -- OTHER INFORMATION

         (1) Name and telephone number of person to contact in regard to this
notification

                Peter Kellogg             (617) 679-2000
--------------------------------------------------------------------------------
                (Name)           (Area Code) (Telephone Number)

         (2) Have all other periodic reports required under Section 13 or 15(d)
of the Securities Exchange Act of 1934 or Section 30 of the Investment Company
Act of 1940 during the preceding 12 months or for such shorter period that the
registrant was required to file such report(s) been filed? If answer is no,
identify report(s). 
                                                                [X] Yes [ ]  No


         (3) Is it anticipated that any significant change in results of
operations from the corresponding period for the last fiscal year will be
reflected by the earnings statements to be included in the subject report or
portion thereof? 
                                                                [X] Yes [ ] No

         If so, attach an explanation of the anticipated change, both
normatively and quantitatively, and, if appropriate, state the reasons why a
reasonable estimate of the results cannot be made.

                  In a press release issued on February 7, 2005 (furnished as
         Exhibit 99.1 of the Company's Current Report on Form 8-K furnished on
         February 7, 2005), the Company announced its unaudited fourth quarter
         and full year 2004 results. These results were announced prior to, and
         do not take into account the impact of, the Company's voluntary
         suspension of TYSABRI or related matters. As noted in Part III of this
         Form, since the February 28, 2005 announcement, the Company has been
         diligently assessing the impact of the suspension of TYSABRI and
         related matters on the Company's 2004 financial statements. The Company
         expects that its audited financial statements for the fiscal year ended
         December 31, 2004 included in its Form 10-K will reflect adjustments to
         the full year 2004 results previously announced on February 7, 2005
         which are primarily related to the TYSABRI suspension and related
         matters as well as additional information. The Company's assessment of
         the impact of the TYSABRI suspension and related matters on its 2004
         financial statements has not been completed as of the date of this
         filing; however, the Company expects that the principal effect of these
         adjustments will be to decrease the Company's 2004 Income Before Income
         Taxes from the amounts previously announced on February 7, 2005 by no
         more than $15 million, resulting in income before income taxes for 2004
         of not less than $78 million.

                  The Company expects that its income from operations and net
         income for 2004 will be substantially greater than for 2003, a year in
         which the Company reported a loss from operations and net loss. This
         increase is expected because of the inclusion in the Company's 2004
         financial statements of a full twelve-months of results of the business
         previously carried on by Biogen, Inc., which merged with a wholly owned
         subsidiary of the Company on November 12, 2003, compared to the
         inclusion in the Company's 2003 financial statements of the results of
         Biogen, Inc. for the period from November 13, 2003 through December 31,
         2003, and increased RITUXAN(R) (rituximab) co-promotion profits in
         2004.



                                Biogen Idec Inc.
--------------------------------------------------------------------------------
                  (Name of Registrant as Specified in Charter)

has caused this notification to be signed on its behalf by the undersigned
hereunto duly authorized.




Date March 17, 2005                     By /s/ Peter N. Kellogg
                                           -------------------------------------
                                           Peter N. Kellogg
                                           Executive Vice President, Finance and
                                           Chief Financial Officer


         INSTRUCTION: This form may be signed by an executive officer of the
registrant or by any other duly authorized representative. The name and title of
the person signing the form shall be typed or printed beneath the signature. If
the statement is signed on behalf of the registrant by an authorized
representative (other then an executive officer), evidence of the
representative's authority to sign on behalf of the registrant shall be filed
with the forms.


                                    ATTENTION

         Intentional misstatements or omissions of fact constitute Federal
criminal violations (see 18 U.S.C. 1001).