Progressive multifocal leukoencephalopathy (PML) is usually a fatal neurological disease. The natural history of PML in patients with human immunodeficiency virus infection was analyzed. The correlations between CD4+ lymphocyte count, previous diagnosis of AIDS, treatment with cytarabine, and survival time are reported for 28 individuals for whom the diagnosis of PML was confirmed by histopathologic examination. For 16 patients (57%), PML was the AIDS-defining illness. For these 16 patients, the mean (+/- SD) survival time after presentation was 7.5 +/- 7.6 months (range, 1-31 months), whereas that for the 12 patients (43%) for whom AIDS was previously diagnosed was 3.2 +/- 2.8 months (range, 1-11 months) (P = .01). The overall mean (+/- SD) CD4+ cell count was 85 +/- 82/mm3 (range, 12-349/mm3). The mean (+/- SD) survival time for patients with CD4+ cell counts of > or = 90/mm3 at the time of presentation was 9.4 +/- 8.7 months, while that for patients with CD4+ cell counts of < 90/mm3 at the time of presentation was 3.6 +/- 1.8 months (P = .03). The nine patients did not benefit from treatment with cytarabine.