To examine the natural history and pathogenesis of parkinsonism in Alzheimer's disease, 44 subjects with clearly established senile dementia of the Alzheimer type were studied during a 66-month period. Sixteen subjects (36%) developed idiopathic parkinsonism, and 12 subjects (27%) developed drug-induced parkinsonism; the chief clinical features of both types were bradykinesia and rigidity, but not resting tremor. The presence of parkinsonism was associated with global (rather than selective) cognitive impairment, as determined by psychometric testing, and with more rapid progression to advanced stages of dementia. The pathological correlates of clinical parkinsonism were heterogeneous in 10 subjects with Alzheimer's disease who were examined post mortem. Coexistent Parkinson's disease was observed in five cases and nonspecific nigral degenerative lesions were present in another three; however, two cases had neither histological changes nor reduced neuronal densities in the substantia nigra. These two cases suggested that extranigral lesions, possibly involving mesocortical dopaminergic pathways, may contribute to the development of parkinsonism in subjects with Alzheimer's disease.