BACKGROUND Immune abnormalities are known to be involved in the pathogenesis of sporadic Parkinson disease. OBJECTIVE To examine whether abnormalities in peripheral lymphocytes exist in Parkinson disease. METHODS Immune mediators, including CD1a, CD3, CD4, CD8, CD45RO, and Fas (CD95), were examined in peripheral lymphocytes of patients by 3-color flow cytometry. RESULTS Patients with Parkinson disease displayed a significantly greater population of circulating CD3+ CD4 bright+ CD8 dull+ lymphocytes than age-matched control subjects (P =.005) and patients with cerebrovascular disease (P =.002). The increase in these cells appeared to continue for at least 17 months. These T cells also expressed CD45RO and Fas, markers for activated T cells, while CD1a, a marker for thymic T cells, was negative, suggesting that these cells are mature T cells with immune activities. CONCLUSIONS As CD4+ CD8+ T cells are known to increase after some specific viral infections, the continuous increase in CD4 bright+ CD8 dull+ T cells shown here may indicate postinfectious immune abnormalities that are possibly associated with the pathogenesis of this slowly progressive, multifactorial neurodegenerative disease.