The etiology of Parkinson's disease is mainly unknown. Immune abnormalities have been described, but the cause of such abnormalities has not been resolved. We examined by two-colour flow cytometry HLA-DR antigen expression on monocytes from cerebrospinal fluid (CSF) and blood and, moreover, lymphocyte subpopulations (CD4+ CD45RO+, CD4+ CD45RA+, CD8+ CD11b+high) in peripheral blood from patients with Parkinson's disease compared with age-matched patients with other neurological diseases (OND) and tension headache. We found higher HLA-DR expression on CSF monocytes compared with blood monocytes. This difference was restricted to Parkinson's disease patients. T helper cell analysis revealed a decreased percentage of CD45RA+“naive” and an increased percentage of CD45RO+“memory” T cell subset from CD4+ T cells in peripheral blood of patients with Parkinson's disease compared with patients with tension headache. The proportions of CD8+ CD11b+high“suppressor” T cells remained unchanged, among the three patient groups compared. A selective loss of CD4+ CD45RA+ cells, previously observed in diseases like multiple sclerosis and Down's syndrome as compared with healthy controls suggests a common immunological abnormality in neurological disorders.