Parkinson disease (PD) is the second most common neurodegenerative disorder 1, 2 . Growing evidence suggests a causative role of misfolded forms of the protein, α-synuclein (αSyn), in the pathogenesis of PD 3, 4 . Intraneuronal aggregates of αSyn occur in Lewy bodies and Lewy neurites 5 , the cytopathological hallmarks of PD and the related disorders called synucleinopathies. αSyn has long been defined as a “natively unfolded” monomer of ∼14 kDa 6 that is believed to acquire α-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 αSyn isolated and analyzed 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 ∼58 kDa. Multiple methods, including analytical ultracentrifugation, scanning transmission electron microscopy and in vivo cell crosslinking, confirmed the occurrence of the tetramer. Native, cell-derived αSyn showed α-helical structure without lipid addition and had much greater lipid binding capacity than the recombinant αSyn studied heretofore. Whereas recombinantly expressed monomers readily aggregated into amyloid-like fibrils in vitro, native human tetramers underwent little or no amyloid-like aggregation. Based on these findings, we propose that destabilization of the helically folded tetramer precedes αSyn misfolding and aggregation in PD and other human synucleinopathies and that small molecules which stabilize the physiological tetramer could reduce αSyn pathogenicity.