AVONEX(R), the Most Prescribed Multiple Sclerosis Therapy Worldwide, Celebrates 10th Anniversary

Anniversary Celebration Includes NASDAQ Ceremonial Bell Ringing,
Educational Events, Commemorative Book and Video

Biogen Idec (NASDAQ:BIIB) today marks the 10th anniversary of
AVONEX(R) (Interferon beta-1a), the most prescribed multiple sclerosis
(MS) therapy worldwide, as the company's CEO James Mullen, rings the
NASDAQ Stock Market Opening Bell. AVONEX, manufactured by Biogen Idec,
was approved for relapsing forms of MS by the U.S. Food and Drug
Administration (FDA) on May 17, 1996. Shortly after its introduction
in the U.S., it became the number one prescribed therapy in MS. Today
more than 130,000 patients worldwide choose AVONEX to treat their MS.
Biogen Idec is recognizing the AVONEX anniversary with the
publication of a commemorative book, "Profiles of Inspiration" and a
companion video. The book and video showcase extraordinary people with
MS whose lives have been positively impacted by AVONEX. The book and
video will soon be available on www.AVONEX.com.
"Biogen Idec is proud to recognize the 10th anniversary of AVONEX,
the number one prescribed MS therapy worldwide, and the impact it has
made on the lives of patients around the world. People living with MS
inspire and motivate us each and every day to redouble our efforts to
advance the science and treatment of this disabling disease. The
introduction of AVONEX was an important step forward in the treatment
of MS, but we know a significant unmet need persists, and we will
continue our unyielding commitment to offer patients hope in their
fight against this disease," said Mullen.
The anniversary is being celebrated by MS patients, physicians and
caregivers in a series of events around the country intended to raise
awareness of MS and to celebrate the role AVONEX has played in
transforming MS care.
"The mission of the National Multiple Sclerosis Society is to end
the devastating effects of MS," said John Richert, MD, vice president,
research and clinical programs of the National Multiple Sclerosis
Society. "We support the efforts of companies such as Biogen Idec who
are committed to research and development of breakthrough therapies
that can offer help to people living with MS."
AVONEX remains the only once-a-week disease modifying therapy
proven to both slow the progression of disability and reduce the
frequency of clinical relapses for people with MS, a chronic disease
of the central nervous system. In the pivotal trial that led to its
approval in the U.S., AVONEX demonstrated a 37% reduction in
progression to disability compared with placebo. Recently published
data demonstrated that AVONEX reduced the risk of developing
clinically definite MS for up to five years in patients who began
treatment immediately after their initial MS attack compared to those
who delayed the treatment.

About AVONEX

AVONEX is the most prescribed treatment for relapsing forms of MS.
It was launched in the U.S. in 1996 and later in Europe for the
treatment of relapsing forms of MS to slow the progression of
disability and reduce relapses. AVONEX is marketed internationally in
more than 90 countries. AVONEX was the first treatment approved for MS
patients who have their first clinical MS attack and have a brain MRI
scan that suggests MS; this use was approved in Europe in 2002 and in
the U.S. in 2003.
The most common side effects associated with AVONEX treatment are
flu-like symptoms including myalgia, fever, fatigue, headache, chills,
nausea, vomiting, pain and asthenia.
AVONEX should be used in caution with patients with depression or
other mood disorders and in patients with seizure disorders. AVONEX
should not be used by pregnant women. Patients with cardiac disease
should be closely monitored. Patients should also be monitored for
signs of hepatic injury. Routine periodic blood chemistry and
hematology tests are recommended during treatment with AVONEX. Rare
cases of anaphylaxis have been reported. Please see complete
prescribing information available at www.AVONEX.com

About Biogen Idec

Biogen Idec creates new standards of care in oncology, neurology
and immunology. As a global leader in the development, manufacturing,
and commercialization of novel therapies, Biogen Idec transforms
scientific discoveries into advances in human healthcare. For product
labeling, press releases and additional information about the company,
please visit www.biogenidec.com.

Safe Harbor/Forward Looking Statements

This press release contains forward-looking statements regarding
AVONEX that are subject to a number of risks and uncertainties. These
statements are based on the companies' current beliefs and
expectation. No forward-looking statement can be guaranteed. Drug
development involves a high degree of risk. Factors which could cause
actual results to differ materially from the companies' current
expectations include: the risk that unexpected concerns may arise from
additional data or analysis or that the company may encounter other
unexpected hurdles. For more detailed information on the risks and
uncertainties associated with Biogen Idec's drug development and other
activities, see the periodic reports of Biogen Idec Inc. filed with
the Securities and Exchange Commission. Biogen Idec assumes no
obligation to update any forward-looking statements, whether as a
result of new information, future events or otherwise.