In patients treated with natalizumab, PML incidence was about 11 cases per 1000 patients with 25 to 48 months of treatment, prior immunosuppressant use, and anti–JC virus antibody–positive status. The incidence was estimated to be 0.09 cases or fewer per 1000 antibody-negative patients. Natalizumab (Tysabri, Biogen Idec and Elan Pharmaceuticals) is approved for the treatment of relapsing–remitting multiple sclerosis in more than 50 countries and also for the treatment of moderate-to-severe Crohn's disease in the United States. As of February 2012, approximately 100,000 patients have been treated with natalizumab worldwide (unpublished data). In the pivotal Natalizumab Safety and Efficacy in Relapsing Remitting Multiple Sclerosis trial (AFFIRM; ClinicalTrials.gov number, NCT00027300), natalizumab monotherapy decreased the risk of progression of disability by 42 to 54% and reduced the annualized rate of relapse by 68%. 1 Natalizumab treatment is associated with the risk of progressive multifocal leukoencephalopathy (PML), . . .