Ownership Submission
FORM 3
 UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

INITIAL STATEMENT OF BENEFICIAL OWNERSHIP OF SECURITIES

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940
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(Print or Type Responses)
1. Name and Address of Reporting Person *
  Cook Anne Marie
 2. Date of Event Requiring Statement (Month/Day/Year)
03/09/2005		 3. Issuer Name and Ticker or Trading Symbol
BIOGEN IDEC INC [BIIB]
(Last)
(First)
(Middle)
14 CAMBRIDGE CENTER		 4. Relationship of Reporting Person(s) to Issuer
(Check all applicable)
_____ Director _____ 10% Owner
__X__ Officer (give title below) _____ Other (specify below)
Acting General Counsel
 5. If Amendment, Date Original Filed(Month/Day/Year)
(Street)

CAMBRIDGE, MA 02142		 6. Individual or Joint/Group Filing(Check Applicable Line)
_X_ Form filed by One Reporting Person
___ Form filed by More than One Reporting Person
(City)
(State)
(Zip)
Table I - Non-Derivative Securities Beneficially Owned
1.Title of Security
(Instr. 4)		 2. Amount of Securities Beneficially Owned
(Instr. 4)		 3. Ownership Form: Direct (D) or Indirect (I)
(Instr. 5)		 4. Nature of Indirect Beneficial Ownership
(Instr. 5)
Common Stock 9,191.9223 (1)
D
 

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly. SEC 1473 (7-02)
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Table II - Derivative Securities Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security
(Instr. 4)		 2. Date Exercisable and Expiration Date
(Month/Day/Year)		 3. Title and Amount of Securities Underlying Derivative Security
(Instr. 4)		 4. Conversion or Exercise Price of Derivative Security 5. Ownership Form of Derivative Security: Direct (D) or Indirect (I)
(Instr. 5)		 6. Nature of Indirect Beneficial Ownership
(Instr. 5)
Date Exercisable Expiration Date Title Amount or Number of Shares
Stock Option (right-to-buy) 09/22/1996(2) 09/22/2005 Common Stock 11,500 $ 11.73 D  
Stock Option (right-to-buy) 12/06/1997(3) 12/06/2006 Common Stock 8,050 $ 16.9 D  
Stock Option (right-to-buy) 12/12/1998(4) 12/12/2007 Common Stock 9,200 $ 15.54 D  
Stock Option (right-to-buy) 12/11/1999(5) 12/11/2008 Common Stock 9,200 $ 35.42 D  
Stock Option (right-to-buy) 12/09/2000(6) 12/09/2009 Common Stock 5,750 $ 62.28 D  
Stock Option (right-to-buy) 04/28/2001(7) 04/28/2010 Common Stock 2,875 $ 49.67 D  
Stock Option (right-to-buy) 12/15/2001(8) 12/15/2010 Common Stock 5,750 $ 45.46 D  
Stock Option (right-to-buy) 12/14/2002(9) 12/14/2011 Common Stock 13,455 $ 49.03 D  
Stock Option (right-to-buy) 12/06/2003(10) 12/06/2012 Common Stock 15,410 $ 37.45 D  
Stock Option (right-to-buy) 02/06/2005(11) 12/06/2014 Common Stock 13,000 $ 43.5 D  
Stock Option (right-to-buy) 02/17/2006(12) 02/17/2015 Common Stock 9,500 $ 67.57 D  

Reporting Owners

Reporting Owner Name / Address Relationships
Director 10% Owner Officer Other
Cook Anne Marie
14 CAMBRIDGE CENTER
CAMBRIDGE, MA 02142		       Acting General Counsel  

Signatures

By: Benjamin S. Harshbarger; For:Anne Marie Cook 03/18/2005
**Signature of Reporting Person Date

Explanation of Responses:

* If the form is filed by more than one reporting person, see Instruction 5(b)(v).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
(1) Number of directly held shares consists of 2291.9223 shares acquired through the Issuer's Employee Stock Purchase Plan, and grants of 4000 and 2900 shares of restricted stock which fully vest on 02/06/2007 and 02/17/2008, respectively.
(2) The stock option became exercisable in six (6) equal annual installments, commencing one year after the grant date of 09/22/95.
(3) The stock option became exercisable in six (6) equal annual installments, commencing one year after the grant date of 12/06/96.
(4) The stock option became exercisable in five (5) equal annual installments, commencing one year after the grant date of 12/12/97.
(5) The stock option became exercisable in five (5) equal annual installments, commencing one year after the grant date of 12/11/98.
(6) The stock option became exercisable in five (5) equal annual installments, commencing one year after the grant date of 12/09/99.
(7) The stock option became exercisable in five (5) equal annual installments, commencing one year after the grant date of 04/28/00.
(8) The stock option became exercisable in five (5) equal annual installments, commencing one year after the grant date of 12/15/00.
(9) The stock option becomes exercisable in four (4) equal annual installments, commencing one year after the grant date of 12/14/01.
(10) The stock option becomes exercisable in four (4) equal annual installments, commencing one year after the grant date of 12/06/02.
(11) The stock option becomes exercisable in four(4) equal annual installments, commencing one year after the grant date of 02/06/04.
(12) The stock option becomes exercisable in four (4) equal annual installments, commencing one year after the grant date of 02/17/05.

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