Measurement of α-synuclein level in the peripheral blood was proposed as a diagnostic test for Parkinson's disease. However, the results of these studies remain contradictory, probably because the examined samples included patients with different etiology of Parkinson's disease. To verify this assumption we studied the levels of α-synuclein in peripheral blood leukocytes of patients with Parkinson's disease associated with mutations in the gene of leucine-rich kinase 2 (LRRK2). The mean α-synuclein level was significantly lower in patients with LRRK2-associated Parkinson's disease (N=8) than in patients with sporadic form of the disease (N=33; p<0.02) and in controls (N=18; p<0.05). On the other hand, we found no differences in the level of α-synuclein level between patients with sporadic form of the disease and controls. We hypothesize that the level of α-synuclein in the peripheral blood largely depends on the etiology of the disease and cannot be used as a universal diagnostic test for Parkinson's disease.