Intracytoplasmic inclusions composed of alpha-synuclein (alpha-syn) are characteristic of neurodegenerative Lewy body disorders. Using novel monoclonal antibodies raised against altered alpha-syn, we uncovered an unprecedented and extensive burden of alpha-syn pathology in the striatum of Lewy body disorders. The highest density of striatal pathology was observed in patients with a combination of Alzheimer's disease and dementia with Lewy bodies or pure dementia with Lewy bodies, and these alpha-syn aggregates may contribute to the parkinsonism seen in these disorders.