The defining neuropathological deposits of Parkinson's disease, dementia with Lewy bodies and multiple system atrophy are strongly immunoreactive for alpha-synuclein. We have shown previously that isolated filaments from dementia with Lewy bodies and multiple system atrophy brains are labelled in a characteristic fashion by a number of alpha-synuclein antibodies. Here we have extracted filaments from substantia nigra of patients with idiopathic Parkinson's disease. Antibodies directed against the carboxy-terminal region of alpha-synuclein labelled isolated filaments along their entire lengths. By contrast, an antibody directed against the amino-terminal region of alpha-synuclein only labelled one filament end. These characteristics were identical to those of filaments extracted from brains of patients with dementia with Lewy bodies and multiple system atrophy.