Parkinson's disease is the second most common neurodegenerative disorder(1,2). Growing evidence indicates a causative role of misfolded forms of the protein alpha-synuclein in the pathogenesis of Parkinson's disease(3,4). Intraneuronal aggregates of alpha-synuclein occur in Lewy bodies and Lewy neurites(5), the cytopathological hallmarks of Parkinson's disease and related disorders called synucleinopathies(4). alpha-Synuclein has long been defined as a 'natively unfolded' monomer of about 14 kDa (ref. 6) that is believed to acquire alpha-helical secondary structure only upon binding to lipid vesicles(7). This concept derives from the widespread use of recombinant bacterial expression protocols for in vitro studies, and of overexpression, sample heating and/or denaturing gels for cell culture and tissue studies. In contrast, we report that endogenous alpha-synuclein isolated and analysed under non-denaturing conditions from neuronal and non-neuronal cell lines, brain tissue and living human cells occurs in large part as a folded tetramer of about 58 kDa. Several methods, including analytical ultracentrifugation, scanning transmission electron microscopy and in vitro cell crosslinking confirmed the occurrence of the tetramer. Native, cell-derived alpha-synuclein showed alpha-helical structure without lipid addition and had much greater lipid-binding capacity than the recombinant alpha-synuclein studied heretofore. Whereas recombinantly expressed monomers readily aggregated intoamyloid-like fibrilsin vitro, native human tetramers underwent little or no amyloid-like aggregation. On the basis of these findings, we propose that destabilization of the helically folded tetramer precedes alpha-synuclein misfolding and aggregation in Parkinson's disease and other human synucleinopathies, and that small molecules that stabilize the physiological tetramer could reduce alpha-synuclein pathogenicity.