Abnormalities of concanavalin A-induced suppressor activity and autologous mixed lymphocyte reaction (which is part of the regulatory role of T cells on lymphocyte functions) were observed in patients with preactive Behçet's disease. However, both the functions returned to normal when the patients entered an active or inactive phase of disease. Thus, a diminution of suppressor function appears to be necessary for the initiation or potentiation of autoimmune and/or inflammatory abnormalities in Behçet's disease. Results of studies of T cell subsets and their functional analysis in patients with preactive Behçet's disease provide further evidence in support of this hypothesis.