Anticardiolipin antibodies(ACA) were assayed by an enzyme-linked immunosorbent assay (ELISA) in 68 patients with Behçet's disease. Twenty seven (39.7 %) patients showed levels of ACA five standard deviations above the value of the control group. The frequency of ACA isotype IgM was found to be significantly increased in these patients. However, ACA was not found to have a significant association with clinical activity, thrombosis, positive Venereal Disease Research Laboratory(VDRL) test or antinuclear antibodies (ANA).