Recently two point mutations in the alpha-synuclein gene have been found in familial Parkinson's disease. The characteristic fibrous neuropathological lesions of Parkinson's and other neurodegenerative diseases have been shown to stain strongly with antibodies against alpha-synuclein and extracted filaments have been labelled with anti-alpha-synuclein antibodies. In view of the close involvement of alpha-synuclein filaments with pathology, it was important to establish an in vitro assembly system. We report here that C-terminally truncated recombinant alpha-synuclein readily assembles into filaments resembling those isolated from diseased brain and suggest that truncation by proteolysis may play a role in the pathological process.