Two mutations in the gene encoding alpha-synuclein have been linked to early-onset Parkinson's disease (PD). alpha-Synuclein is a component of Lewy bodies, the fibrous cytoplasmic inclusions characteristic of nigral dopaminergic neurons in the PD brain. This connection between genetics and pathology suggests that the alpha-synuclein mutations may promote PD pathogenesis by accelerating Lewy body formation. To test this, we studied alpha-synuclein folding and aggregation in vitro, in the absence of other Lewy body-associated molecules. We demonstrate here that both mutant forms of alpha-synuclein (A53T and A30P) are, like wild-type alpha-synuclein (WT), disordered in dilute solution. However, at higher concentrations, Lewy body-like fibrils and discrete spherical assemblies are formed; most rapidly by A53T. Thus, mutation-induced acceleration of alpha-synuclein fibril formation may contribute to the early onset of familial PD.