It is not known whether an increased incidence of dementia in patients with Parkinson's disease (PD) is due to a higher incidence of Alzheimer's disease (AD) or to "early" Alzheimer-type pathology. To determine whether amyloid beta-peptide (A beta) of AD occurs more frequently in brains of patients with PD, we examined 50 cases and 79 controls by using histoblots for A beta. Twenty-three cases with PD had dementia, including all nine with A beta distributed throughout the entire cerebral cortex; three of these cases had AD. In contrast, five of 17 controls with comparable A beta accumulation were not demented. Neither AD nor A beta deposition was increased in PD, furthermore, there was no statistical correlation between the amount of A beta and the number of Lewy bodies in cerebral cortex. In 14 patients with PD in whom dementia was unrelated to A beta, there was cerebral vascular disease (four), numerous cortical Lewy bodies (three), or hydrocephalus (two); in five further cases, dementia was not well explained by histopathologic changes. Our data found no increase of either AD or "early" Alzheimer-type pathology in cases of PD; however, a synergistic effect between the two pathologies was suggested as contributing to dementia.