Biogen Idec Prices $1.0 Billion Senior Unsecured Notes Offering

Biogen Idec Inc. (NASDAQ: BIIB) announced today that it has priced
a public offering of $1.0 billion principal amount of senior unsecured
notes. The offering of senior unsecured notes includes $450 million in
aggregate principal amount of 6.0% notes due 2013 and $550 million in
aggregate principal amount of 6.875% notes due 2018. The sale of the
notes is expected to close on March 4, 2008, subject to customary
closing conditions.

Biogen Idec plans to use the net proceeds from this offering,
together with cash on hand, to repay indebtedness under its $1.5
billion bridge facility, the proceeds of which were used to repurchase
shares of Biogen Idec´s common stock in a $3.0 billion "Dutch Auction"
tender offer settled on July 2, 2007.

Goldman, Sachs & Co. and Merrill Lynch & Co. are the book-running
managers for the offering. The offering of these securities is being
made only by means of a prospectus and related prospectus supplement.
Electronic copies of the prospectus and related prospectus supplement
may be requested from Goldman, Sachs & Co. by mail at Goldman, Sachs &
Co., Attention:

Prospectus Department, 85 Broad Street, New York, NY 10004, by fax
at (212) 902-9316 or by email at prospectus-ny@ny.email.gs.com or by
Merrill Lynch, Pierce, Fenner & Smith Incorporated at 4 World
Financial Center, New York, New York 10080, Attention: Prospectus
Department, 866-500-5408.

About Biogen Idec

Biogen Idec creates new standards of care in therapeutic areas
with high unmet medical needs. Founded in 1978, Biogen Idec is a
global leader in the discovery, development, manufacturing, and
commercialization of innovative therapies. Patients in more than 90
countries benefit from Biogen Idec´s significant products that address
diseases such as lymphoma, multiple sclerosis, and rheumatoid
arthritis. For product labeling, press releases and additional
information about the company, please visit www.biogenidec.com.

Safe Harbor

This press release contains forward-looking statements.
Forward-looking statements are subject to risks and uncertainties that
could cause actual results to differ materially from that which we
expect. Important factors that could cause our actual results to
differ include our continued dependence on our two principal products,
AVONEX(R) and RITUXAN(R), the uncertainty of success in
commercializing other products including TYSABRI(R), the occurrence of
adverse safety events with our products, the consequences of the
nomination of directors for election to our Board by an activist
shareholder, the failure to execute our growth strategy successfully
or to compete effectively in our markets, our dependence on
collaborations over which we may not always have full control,
possible adverse impact of government regulation and changes in the
availability of reimbursement for our products, problems with our
manufacturing processes and our reliance on third parties,
fluctuations in our operating results, our ability to protect our
intellectual property rights and the cost of doing so, the risks of
doing business internationally and the other risks and uncertainties
that are described in Item 1.A. Risk Factors in our reports on Form
10-K and Form 10-Q and in other periodic and current reports we file
with the SEC. These forward-looking statements speak only as of the
date of this press release, and we do not undertake any obligation to
publicly update any forward-looking statements, whether as a result of
new information, future events, or otherwise.