Alpha-synuclein (alpha-syn) is an intracellular protein with a high tendency to aggregation. It is the major component of Lewy bodies and may play a key role in the pathogenesis of Parkinson's disease (PD). alpha-Syn is also released by neurons and can be detected in biological fluids, such as plasma. The purpose of this study was to determine whether plasma alpha-syn concentrations are elevated in newly diagnosed PD patients before treatment (nontreated PD group, ntPD; n = 53) and to compare them with concentrations in PD patients with at least 1 year of specific treatment (tPD; n = 42) and in healthy controls (n = 60). Plasma alpha-syn concentrations in the ntPD and tPD groups were similar and significantly higher than in healthy controls. In conclusion, alpha-syn was elevated early in the development of PD and specific PD treatment did not change plasma alpha-syn levels.