Intracytoplasmic inclusions composed of α‐synuclein (α‐syn) are characteristic of neurodegenerative Lewy body disorders. Using novel monoclonal antibodies raised against altered α‐syn, we uncovered an unprecedented and extensive burden of α‐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 α‐syn aggregates may contribute to the parkinsonism seen in these disorders.