The February 14th issue of The Medical Letter reviewed natalizumab (Tysabri - Biogen Idec), a new monoclonal antibody that was granted accelerated approval because it decreased the number of relapses in patients with multiple sclerosis. On February 28th, the FDA issued a Public Health Advisory announcing that marketing of the drug had been suspended because 2 patients who had been treated with natalizumab for more than 2 years had developed progressive multifocal leukoencephalopathy, a rare, often fatal disease (www.fda.gov).