Abnormal biology of alpha-synuclein (alpha-Syn) is directly implicated in the pathogenesis of Parkinson's disease and other alpha-synucleinopathies. Herein, we demonstrate that C-terminally truncated alpha-Syn (alpha-SynDeltaC), enriched in the pathological alpha-Syn aggregates, is normally generated from full-length alpha-Syn independent of alpha-Syn aggregation in brains and in cultured cells. The accumulation of alpha-SynDeltaC is enhanced in neuronal cells as compared with nonneuronal cells. Significantly, the expression of familial Parkinson's disease-linked mutant alpha-Syn is associated with the enhanced cellular accumulation of alpha-SynDeltaC. Moreover, substoichiometric amounts of alpha-SynDeltaC enhance the in vitro aggregation of the more abundant full-length alpha-Syn. Finally, cases of alpha-synucleinopathy exhibit increases in the total soluble alpha-Syn and a higher proportion of soluble alpha-SynDeltaC, a condition favoring the aggregation of alpha-Syn. Collectively, our results indicate that the biology behind the generation and accumulation of alpha-SynDeltaC is likely to have relevance for the initiation and the progression of alpha-Syn aggregation in vivo.