Immune reaction-related inflammation may be important in the pathogenesis of Parkinson disease (PD). To elucidate peripheral immunologic alterations in PD, we characterized extended peripheral T-lymphocyte populations in 33 patients with PD and 34 normal subjects. Patients with PD had significantly decreased CD4⁺:CD8⁺T-cell ratios (P<0.001), fewer CD4⁺CD25⁺T cells (P<0.01), and significantly increased ratios of IFN-γ-producing to IL-4-producing T cells (P<0.001). The characteristics of predominant expression of CD8⁺T cells, depletion of CD4⁺CD25^high cells, and a shift to a T_H1-type immune response in the peripheral immune system in PD patients may reflect an immune reaction-associated inflammatory process in the brain.