Because patients with Alzheimer's disease often develop clinical manifestations of Parkinson's disease, we examined the substantia nigra in 40 cases of pathologically confirmed Alzheimer's disease for the changes of Parkinson's disease (neuronal loss, Lewy bodies, or neurofibrillary tangles). Eighteen patients had one or more of these changes in the substantia nigra. Subsequently, we reviewed their clinical records and found that rigidity, with or without tremor, had been noted in 13 patients, and nine patients had a second diagnosis of possible or definite Parkinson's disease. Eleven (85%) of these patients had the pathologic changes of Parkinson's disease. These findings suggest that the majority of patients with Alzheimer's disease with extrapyramidal signs have the pathologic changes of Parkinson's disease in the substantia nigra.