Several immunological abnormalities have been described in Behçet's syndrome, the multisystem disease characterized by the triad of relapsing iridocyclitis with recurrent oral and genital ulcerations. In the present study we have evaluated T cell subsets in the peripheral blood of patients suffering from Behçet's syndrome, using a panel of anti-T cell monoclonal antibodies. When compared with normal subjects, patients with Behçet's syndrome show a discrepancy between the number of T3+ cells and those forming rosettes with sheep erythrocytes, a significantly higher number of T4+ and T8+ double labelled cells, as well as of T6+ lymphocytes in the peripheral blood. The percentage of T8 T lymphocytes is increased, thus lowering the T4+/T8+ cell ratio. The results of this study indicate that a complex imbalance of T cell subsets is present in Behçet's syndrome and give a rationale for possible treatment of these patients with immunomodulators.