Progressive multifocal leukoencephalopathy, a formerly rare disease that chiefly occurred in persons with underlying lymphoma and chronic lymphocytic leukemia, is now seen with increasing frequency in the era of acquired immunodeficiency syndrome. Progressive multifocal leukoencephalopathy is currently estimated to arise in 5% of all human immunodeficiency virus-infected individuals. The clinical features of the disorder in patients with acquired immunodeficiency syndrome do not appear to be significantly different from progressive multifocal leukoencephalopathy occurring in association with other immunosuppressive disorders. Radiographically, the appearance of HIV dementia on magnetic resonance imaging is sometimes confused with that of progressive multifocal leukoencephalopathy. Among the characteristics that are helpful in distinguishing between the two disorders are the presence of focal findings, the rate of disease progression, the specific magnetic resonance imaging attributes, including the location of the lesions, and certain cerebrospinal fluid parameters, including surrogate markers for human immunodeficiency virus dementia and the presence of myelin basic protein. The remarkable increase in the burden of progressive multifocal leukoencephalopathy has provided a vital impetus for its study, particularly with respect to diagnosis and therapy. Establishing an unequivocal diagnosis of progressive multifocal leukoencephalopathy currently requires brain biopsy. The application of polymerase chain reaction for JC virus amplification to cerebrospinal fluid samples suggests that it may provide an alternative means of diagnosis. Recent in vitro studies of cytosine arabinoside and camptothecin suggest that they, or similar agents, may prove useful in the treatment of this illness and well-designed clinical trials are underway.