Phase II Clinical Trial Results for ADENTRI Published in Journal of the American College of Cardiology

Biogen Idec (NASDAQ: BIIB) has announced that positive results of
a Phase II study of oral ADENTRI(R), an A1 adenosine receptor
antagonist, in stable heart failure patients have been published in
today's issue of the Journal of the American College of Cardiology.
Results showed that administration of oral ADENTRI for 10 days, in
addition to standard heart failure therapy, was well tolerated and
resulted in clinically significant increases in sodium excretion while
preserving renal function.

"We are excited to have these data published showing the potential
of ADENTRI in patients with heart failure. Worsening kidney function
occurs commonly in these patients and it is typically associated with
adverse outcomes," said Barry H. Greenberg, MD, Professor of Medicine
and Director of the Advanced Heart Failure Program at University of
California, San Diego Medical Center. "Our results demonstrate
significant increases in salt excretion without significant adverse
events on renal function, suggesting that ADENTRI may have the
potential to improve the standard of treatment that is currently
offered."

According to the American Heart Association (AHA), there are
currently five million people that suffer from heart failure in the
United States. In this patient population studies have demonstrated
that renal function is one of the most important independent
determinants of survival. Heart failure patients with renal
insufficiency are also considered to be among the most challenging to
treat. It is believed that worsened renal function is caused by a
combination of pre-existing medical conditions and the chronic use of
high-dose diuretics.

"Biogen Idec is committed to developing novel compounds for
patients with cardiopulmonary diseases with high unmet need," said
Barry Ticho, M.D., Ph.D, Senior Director of Medical Research at Biogen
Idec. "We are moving forward with oral and intravenous formulations of
ADENTRI for acute and chronic heart failure, as well as two additional
compounds; LIXIVAPTAN for the treatment of hyponatremia in congestive
heart failure and AVIPTADIL for the treatment of pulmonary arterial
hypertension."

The trial was a randomized, double blind, placebo-controlled study
that sought to assess the pharmacokinetics and pharmacologic effects
of ADENTRI in heart failure patients.

Fifty patients were maintained on their usual medications,
including ACE inhibitors and diuretics, and were dosed with placebo or
one of four doses of ADENTRI, administered once daily for 10 days.

The study showed increases in sodium excretion above baseline and
placebo beginning on Day 1, continuing over the 10-day dosing period.
Notably, these effects were not accompanied by reductions in renal
function nor substantial increases in potassium excretion. Trends
toward beneficial effects in clinical measures of heart failure,
including body weight, edema, and physician global assessment, were
also observed. The safety evaluation did not reveal any significant
concerns during dosing nor during 30 days of additional follow-up. The
incidence of adverse events was similar to placebo, and higher doses
of ADENTRI were not associated with an increase in adverse events. The
pharmacokinetic profile was consistent with once daily dosing.

Preliminary results of this study were presented in 2003 at the
annual meeting of the American Heart Association.

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/Forward Looking Statements

This press release contains forward-looking statements regarding
ADENTRI(R). These statements are based on Biogen Idec's current
beliefs and expectations. The commercial potential of ADENTRI is
subject to a number of risks and uncertainties, including the risk of
unexpected delays or hurdles and the uncertainty of obtaining
regulatory approval. Drug development and commercialization involves a
high degree of risk.

For more detailed information on the risks and uncertainties
associated with Biogen Idec's drug development and other activities,
see Item 1A "Risk Factors" in Biogen Idec's most recent Form 10-Q
filing with the Securities and Exchange Commission. These forward
looking statements speak only as of the date of this press release,
and we assume no obligation to update any forward-looking statements,
whether as a result of new information, future events or otherwise.