Three patients have developed progressive multifocal leukoencephalopathy while being treated with natalizumab for their autoimmune diseases. One patient had relapsing-remitting multiple sclerosis (RRMS), one had Crohn's disease, and one (who had been enrolled in an RRMS phase III trial) had no evidence of multiple sclerosis on autopsy. The patient with RRMS now has severe, permanent neurologic damage and the other two patients died. In this article, we recommend revised entry and diagnostic criteria to prevent enrollment of RRMS patients with a favorable prognosis or questionable diagnosis into trials of drugs with unknown safety profiles. In light of the risk of progressive multifocal leukoencephalopathy, we strongly recommend that RRMS patients who are at low risk of disability not be treated with natalizumab. Finally, we discuss what additional natalizumab efficacy data need to be presented before any decisions should be made about treating RRMS patients with a high risk of developing long-term disability.