SEC Form 4

FORM 4		UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940	OMB APPROVAL OMB Number: 3235-0287 Estimated average burden hours per response: 0.5
	Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

1. Name and Address of Reporting Person* ROHN WILLIAM R (Last) (First) (Middle) C/O CERUS CORP 2411 STENWELL DRIVE (Street) CONCORD CA 94520 (City) (State) (Zip)	2. Issuer Name and Ticker or Trading Symbol BIOGEN IDEC INC [ BIIB ]	5. Relationship of Reporting Person(s) to Issuer (Check all applicable) Director 10% Owner X Officer (give title below) Other (specify below) Exec VP & COO
	3. Date of Earliest Transaction (Month/Day/Year) 02/20/2004
	4. If Amendment, Date of Original Filed (Month/Day/Year)	6. Individual or Joint/Group Filing (Check Applicable Line) X Form filed by One Reporting Person Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1. Title of Security (Instr. 3)	2. Transaction Date (Month/Day/Year)	2A. Deemed Execution Date, if any (Month/Day/Year)	3. Transaction Code (Instr. 8)		4. Securities Acquired (A) or Disposed Of (D) (Instr. 3, 4 and 5)			5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4)	6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	7. Nature of Indirect Beneficial Ownership (Instr. 4)
			Code	V	Amount	(A) or (D)	Price
Common Stock	02/18/2004		M		18,100	A	$6.1875	37,725	D
Common Stock	02/18/2004		S(1)		18,100	D	$51.9	19,625	D
Common Stock	02/18/2004		M		4,400	A	$6.1875	24,025	D
Common Stock	02/18/2004		S(1)		4,400	D	$51.91	19,625	D
Common Stock	02/18/2004		M		500	A	$6.1875	20,125	D
Common Stock	02/18/2004		S(1)		500	D	$51.938	19,625	D
Common Stock	02/18/2004		M		1,500	A	$6.1875	21,125	D
Common Stock	02/18/2004		S(1)		1,500	D	$51.92	19,625	D
Common Stock	02/18/2004		M		500	A	$6.1875	20,125	D
Common Stock	02/18/2004		S(1)		500	D	$51.94	19,625	D
Common Stock								6,000	I	by Spouse
Common Stock(2)								377,388	I	by Trust

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 3)	2. Conversion or Exercise Price of Derivative Security	3. Transaction Date (Month/Day/Year)	3A. Deemed Execution Date, if any (Month/Day/Year)	4. Transaction Code (Instr. 8)		5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5)		6. Date Exercisable and Expiration Date (Month/Day/Year)		7. Title and Amount of Securities Underlying Derivative Security (Instr. 3 and 4)		8. Price of Derivative Security (Instr. 5)	9. Number of derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)	10. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	11. Nature of Indirect Beneficial Ownership (Instr. 4)
				Code	V	(A)	(D)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares
Incentive Stock Option (right to buy)(2)	$6.1875	02/18/2004		M			18,100	04/15/1999(3)	04/14/2008	Common Stock	18,100	(2)	80,175	D
Incentive Stock Option (right to buy)(2)	$6.1875	02/18/2004		M			4,400	04/15/1999(3)	04/14/2008	Common Stock	4,400	(2)	75,775	D
Incentive Stock Option (right to buy)(2)	$6.1875	02/18/2004		M			500	04/15/1999(3)	04/14/2008	Common Stock	500	(2)	75,275	D
Incentive Stock Option (right to buy)(2)	$6.1875	02/18/2004		M			1,500	04/15/1999(3)	04/14/2008	Common Stock	1,500	(2)	73,775	D
Incentive Stock Option (right to buy)(2)	$6.1875	02/18/2004		M			500	04/15/1999(3)	04/14/2008	Common Stock	500	(2)	73,275	D

Explanation of Responses:

1. Represents sale by William Rohn pursuant to a qualified written selling plan under SEC rule 10b5-1.

2. Granted under Issuer's Employee Stock Option Plan, in an exempt transaction under SEC rule 16b-3(d).

3. Option became exercisable as to 25% of the optioned shares on 4/15/99 and as to the balance of the shares in 36 equal monthly installments thereafter.

	By: Pamela A. Blas For: William R. Rohn	02/20/2004
	** Signature of Reporting Person	Date
Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
* If the form is filed by more than one reporting person, see Instruction 4 (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).
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