Before the AIDS epidemic, progressive multifocal leukoencephalopathy (PML) was a rare disorder occurring most often in association with leukemia and lymphoma. Current estimates indicate that PML ultimately develops in up to 5% of all patients with AIDS. This demyelinating disease results from infection with JC virus, a papova virus, that most of the world's population is exposed to prior to adulthood. Although PML commonly occurs in the setting of advanced immunosuppression, it may be observed in patients with CD4 lymphocyte counts in excess of 200 cells/mm3. Focal neurological symptoms and signs coupled with hyperintense signals abnormalities of the white matter on T2-weighted cranial magnetic resonance imaging are highly suggestive of the disease. In this setting, a positive CSF polymerase chain reaction for JCV DNA has been felt to be sufficiently diagnostic to eliminate the need for brain biopsy. Survival of AIDS-associated PML is poor with median survivals averaging just 6 months. However, as many as 10% of AIDS patients with PML will have prolonged (>12 months) survival and partial recovery. Highly active antiretroviral therapy (HAART) has been demonstrated to have a salutary effect on survival.