Progressive multifocal leukoencephalopathy (PML) complicating the acquired immunodeficiency syndrome (AIDS) is typically inexorably progressive with death usually occurring within 6 months of symptom onset. Occasional patients have been observed to survive longer than 1 year, often with remission of clinical features. In this study, we identify predictive factors for prolonged survival in patients with biopsy proven, AIDS-associated PML, by comparing 7 patients with survival exceeding 12 months from symptom onset with 45 patients with shorter survivals. PML was the presenting manifestation of AIDS in 5 (71.4%) of 7 long-term survivors compared with 8 (17.8%) of 45 short-term survivors. CD4 T-lymphocyte counts were substantially higher in the long-term survivors, with 3 (42.9%) of 7 having counts exceeding 300 cells/mm3 in comparison with only 1 (4.3%) of 23 short-term survivors. Contrast enhancement on radiographic imaging was observed in 3 (50%) of 6 long-term survivors in comparison with 4 (8.9%) of 45 short-term survivors. Neurological recovery and radiographic improvement were not observed in any short-term survivors but were seen in 5 (71.4%) long-term survivors. There was no association between treatment modalities and survival. Predictors of long-term survival in AIDS patients with PML include PML as the heralding manifestation of AIDS, high CD4 T-lymphocyte count at disease onset, lesion enhancement on computed tomographic scan or magnetic resonance imaging, and evidence of recovery of neurological function.