We evaluated a 66-year-old man with a rapidly progressive, akinetic-rigid dementia syndrome. Despite extensive testing, which included magnetic resonance imaging (MRI), we were unable to make the correct antemortem diagnosis. Autopsy demonstrated spontaneous progressive multifocal leukoencephalopathy. This report illustrates that even in the absence of characteristic MRI findings, this uncommon cause of dementia should be considered in the differential diagnosis of rapidly progressive, akinetic-rigid syndromes with dementia.