We report an autopsy case of Parkinson's disease mimicking senile dementia of the Alzheimer type. A Japanese man developed memory disturbance and visual hallucination at age 70. Although he died from pneumonia at age of 74, he had no neurological signs throughout the clinical course. The weight of his brain was 1,420 g. Macroscopic examination of the brain revealed prominent depigmentation of the substantia nigra and locus ceruleus. Histological examination disclosed neuronal loss with astrocytosis and the appearance of the Lewy bodies in the nucleus basalis of Meynert, substantia nigra, locus ceruleus, and dorsal vagal nucleus. There were widespread senile plaques in the brain, including the precentral gyrus, which was compatible with Braak stage C. A small number of neurofibrillary changes were present in the limbic areas, consistent with Braak stage III. This case is consistent with brain stem dominance with the pathological diagnosis of the Consortium on Dementia with Lewy Bodies International Workshop. That is, it is compatible with Parkinson's disease. We postulate that the clinical features of Parkinson's disease are more widespread than previously considered.