Progressive multifocal leukoencephalopathy, a common complication of infections with human immunodeficiency virus (HIV), occurs in as many as 3.8% of patients with the acquired immunodeficiency syndrome (AIDS). We report 16 cases and review 12 previously reported cases of progressive multifocal leukoencephalopathy associated with HIV infection. This illness was the presenting manifestation of HIV infection in 8 cases. Limb weakness, gait abnormalities, visual loss, aand altered mental status were the commonest initial complaints. Computed tomography of the brain frequently showed hypodense, nonenhancing white matter lesions. Magnetic resonance imaging was more sensitive than computed tomography in detecting lesions. Cerebrospinal fluid analysis and electroencephalography were nondiagnostic. Impaired cell-mediated immunity was typically noted, even in the absence of other immunodeficiency-associated illnesses. Death occurred within 10 days to 18 months of the onset of symptoms in 22 patients. However, 4 patients remain alive at 3 to 23 months; of these 4, 2 have had significant improvement without treatment. Various therapies were unsuccessful.