α-Synuclein forms the major component of Lewy bodies and Lewy neurites, the defining neuropathological characteristics of Parkinson's disease and dementia with Lewy bodies. Here we show that α-synuclein is also the major component of the filamentous inclusions of multiple system atrophy which comprises several neurodegenerative diseases with a shared filamentous pathology in nerve cells and glial cells. These findings provide an unexpected link between multiple system atrophy and Lewy body disorders and establish that α-synucleinopathies constitute a major class of human neurodegenerative disorder.