Parkinson's disease is neuropathologically defined by the presence of intracytoplasmic Lewy bodies and Lewy neurites. Similar filamentous inclusions are also present in dementia with Lewy bodies, a common late-life dementia, which is clinically similar to Alzheimer's disease. Lewy bodies and Lewy neurites are made of abnormal filamentous material containing alpha-synuclein. Glial cytoplasmic inclusions in multiple system atrophy also contain alpha-synuclein. Using recombinant alpha-synuclein, we have produced synthetic filaments indistinguishable from those present in the intracellular inclusions of Parkinson's disease, dementia with Lewy bodies and multiple system atrophy. Taken together, these findings indicate that Parkinson's disease, dementia with Lewy bodies and multiple system atrophy are alpha-synucleinopathies.