Since 1980, three immunocompromised patients have been proved to have progressive multifocal leukoencephalopathy (PML) by brain biopsy at the University of Alabama at Birmingham. Two patients presented with focal neurological findings, and the third presented with dementia. Computed tomography (CT) revealed white matter low density lesions in areas appropriate to the neurological abnormalities. Brain biopsy of areas that were abnormal on CT produced diagnostic tissue in all three patients. No patient suffered ill effects from the biopsy. Neuropathological findings on light microscopy were compatible with PML in each case, although there was diversity within the group. Involvement of gray and white matter was present in all biopsy specimens; oligodendrocytes, astrocytes, and neurons were affected. Electron microscopic demonstration of particles compatible with polyoma virus confirmed the diagnosis in each case. Immunosuppressive medication was discontinued in two of the patients; these two have survived more than 2 years after diagnosis. One of these two has gradually improved and is independent in simple activities of daily life. Brain stem and cerebellar involvement and seizure disorders have been present in all reported cases. PML can be accurately and rapidly diagnosed by brain biopsy, enabling therapeutic manipulations that may prolong survival.