The etiology of Parkinson's disease is mainly unknown. Immune abnormalities have been reported, including the occurrence of autoantibodies against neuronal structures and abnormal T cell functions. gamma delta+ T cells represent a recently recognized T cell subpopulation which is considered to play a role in immune responses in infections and autoimmunity. We examined by three-colour flow cytometry the proportions of gamma delta+ T cells in blood and cerebrospinal fluid (CSF) from patients with Parkinson's disease. Increased proportions of gamma delta+ T cells were found in CSF in Parkinson's disease compared to other neurological diseases (OND) and tension headache. In peripheral blood, patients with Parkinson's disease had higher numbers of gamma delta+ T cells compared to OND patients. No differences between gamma delta+ T cells in CSF compared to blood were demonstrable in the individual patient groups. CD25 was not expressed on gamma delta+ T cells in blood of the majority of cases, but 50% of patients with Parkinson's disease and 30% with OND and tension headache had CD25+ gamma delta+ T cells in CSF arguing for a preferential activation of gamma delta+ T cells in the CSF compartment. Whether the elevated gamma delta+ T cell population in Parkinson's disease reflects previously unrecognized inflammation or may occur also in non-inflammatory disorders remains to be elucidated.