To study the immunopathogenesis of neuro-Behçet syndrome, we performed serial cerebrospinal fluid (CSF) examinations in a patient with Behçet's syndrome and involvement of the central nervous system. Before and after immunosuppressive treatment, we measured the CSF indexes of immunoglobulins (Ig), and the third (C3) and the fourth (C4) component of complement, and quantified immune complexes and lymphocyte subsets in CSF and peripheral blood. During active encephalitis, humoral abnormalities were intrathecal production of IgM and, to a lesser degree, IgG and IgA, presence of immune complexes in CSF but not in peripheral blood, intrathecal C3 production, and elevated CSF C3 and C4 concentrations; lymphocyte subset analysis showed an increased CSF CD8+ T-cell percentage, in combination with slightly increased PB CD3+ and CD8+ T-cell subsets. After effective immunosuppressive treatment, humoral and cellular CSF values were normal. We conclude that intrathecally produced immunoglobulins, immune complexes, and C3 as well as CD8+ T cells are likely to participate in the development of Behçet encephalitis.