Thirty-four autopsy cases conforming to the standard neuropathologic criteria of Parkinson disease were sex- and age-matched with controls who had died of infarct or trauma. All brains were reviewed for changes compatible with Alzheimer disease, and available clinical data were retrospectively reviewed. Nineteen (56 percent) of the Parkinson cases had shown some degree of dementia. The average parkinsonian brain weight was 1281 gm; it was 1365 gm for the controls (p < 0.02). Plaques, neurofibrillary tangles, granulovacuolar degeneration, and cortical cell loss were present in all but one of the parkinsonian brains; these pathologic changes were present in fewer controls and to a lesser degree. The higher incidence of dementia in patients with Parkinson disease may be explained by the simultaneous presence of Alzheimer disease.