The morphological differentiation of Parkinson disease with dementia (PDD) and dementia with Lewy bodies (DLB) is a matter of discussion. The objective of this study was to investigate the regional distribution of beta-amyloid (Abeta) plaques, alpha-synuclein (AS), and pathology in both disorders. The basal ganglia from 17 age-matched patients of PDD and DLB each were immunohistochemically examined with variable degrees of associated Alzheimer pathology using antibodies to Abeta, AS, and tau. DLB brains showed a significantly higher burden of (diffuse) amyloid plaques in the putamen and caudate nucleus and slightly more severe tau pathology than PDD brains despite similar neuritic Braak stages. Phases of Abeta development in DLB brains often, but inconsistently, correlated with both neuritic Braak stages and severity of striatal Abeta load, while these correlations were almost never seen in PDD cases with Alzheimer lesions. They also revealed a higher burden of AS-lesions (both Lewy neurites and Lewy bodies) than PDD cases that commonly had a paucity of all three types of lesion. The globus pallidus was virtually spared in both phenotypes. Differences in AS and Abeta pathologies and much less of tau lesions in the striatum support a morphologic distinction between PDD and DLB, which may be of pathophysiologic importance, but the causes of these differences are unclear.