Parkinson's disease (PD) is one of many neurodegenerative diseases that are characterized by amyloid fibril formation. Alpha-synuclein is a primary component of the fibrillar neuronal inclusions, known as Lewy bodies, that are diagnostic of PD. In addition, the alpha-synuclein gene is linked to familial PD. Fibril formation by alpha-synuclein proceeds via discrete beta-sheet-rich oligomers, or protofibrils, that are consumed as fibrils grow. Both FPD mutations accelerate formation of protofibrils, suggesting that these intermediates, rather than the fibril product, trigger neuronal loss. In idiopathic PD, other factors may be responsible for accelerating protofibril formation by wild-type alpha-synuclein. One possible factor could be molecular crowding in the neuronal cytoplasm. We demonstrate here that crowding using inert polymers significantly reduced the lag time for protofibril formation and the conversion of the protofibril to the fibril, but did not affect the morphology of either species. Physiologically realistic changes in the degree of in vitro crowding have significant kinetic consequences. Thus, nonspecific changes in the total cytoplasmic protein concentration, induced by cell volume changes and/or altered protein degradation, could promote formation of and stabilize the alpha-synuclein protofibril.