Atypical forms of progressive multifocal leukoencephalopathy (PML) may simulate other disorders. A previously healthy 70-year-old female developed unsteadiness of gait, dysarthria, dementia and weakness leading to inanition and death from bronchopneumonia over a 43 month period. The diagnosis of PML was not suspected prior to death. Neuropathologic examination of the brain disclosed characteristic findings of PML-deep bilateral cerebral demyelinative foci with enlarged gemistocytic astrocytes and swollen oligodendrocytes containing intranuclear inclusions. Electron microscopy identified papova virus particles within these inclusions. An underlying source of immunosuppression was not identified either premortem nor at the time of autopsy. The prolonged clinical course, simulating that of a primary degenerative disease, and the lack of apparent immunocompromise are unusual features of PML and lend credence to the suggestions that variations in its expression and course are to be expected.