An autopsy case with clinical features of progressive parkinsonism and dementia of presenile occurrence was characterized by the following neuropathological findings: (1) severe degeneration in the striatonigral system and (2) widespread occurrence of numerous senile plaques and Alzheimer's neurofibrillary tangles in the cerebral cortex. We believe this is to be a very rare case of striatonigral degeneration combined with Alzheimer's disease, because, as far as we know, only two similar cases have been reported and they had not such typical characteristics of Alzheimer's disease as ours did. In addition, in our case the biochemical examination revealed that the activity of one of the dopamine-synthesizing enzymes, tyrosine hydroxylase, was greatly decreased in the nigro-striato-pallidal system.