Behçet's disease is a chronic, multisystem disorder characterized by a recurrent inflammatory reaction. Antiendothelial cell antibodies have been detected in the serum from patients with autoimmune diseases with presenting vasculitis and it is assumed that they can induce damage to the endothelial cells. In this study, we detected antiendothelial cell antibodies in the serum from patients with Behçet's disease using an enzyme-linked immunosorbent assay, electrophoresis and immunoblotting. The cytolysis of human dermal microvascular endothelial cells (HDMEC) was measured using a cytotoxicity assay. The serum from 37.4% of Behçet's disease patients showed IgM antibodies against unstimulated HDMEC while the serum from 18.4% of patients showed an increase in IgM antibody titer after IFN-gamma pretreatment. The frequency of vasculitis was higher in the IgM-positive Behçet's disease patients than in the IgM-negative patients. In Western blotting, IgM-positive Behçet's disease serum reacted with the 44 kDa HDMEC surface antigen, whereas IgM-positive systemic lupus erythematosus serum reacted with the 81 kDa HDMEC surface antigen. The reactivity to the 44 kDa protein band was also observed in cultured human umbilical vein endothelial cells but not in fibroblasts, A431 cells or SK-MEL-2 cells. Serum from Behçet's disease patients incubated with human complement or mononuclear cells produced no significant lysis of HDMEC, and cultured HDMEC were resistant to antibody-dependent cell-mediated cytotoxicity. The results suggest that antibodies against antigens on the surface of endothelial cells may play a role in inducing vasculitis in Behçet's disease, not through a direct toxic effect of an antiendothelial cell antibody but by an indirect effect involving the activation of endothelial cells to produce cytokines.