Reduced fibrinolytic activity has frequently been reported in Behcet's syndrome, but both the underlying mechanism and its relation to the development of thrombosis are unclear. The fibrinolytic activity was studied in seven patients with Behçet's syndrome and 12 patients with idiopathic oral ulceration. The patients with Behçet's disease had significantly reduced resting tissue plasminogen activator(t-PA) both by euglobulin fibrin plate lysis and enzyme linked immunosorbent assay (ELISA), but the tissue plasminogen activator inhibitor(t-PAI) was not significantly different from the control value. After stimulation by venous occlusion the euglobulin fibrin plate lysis results showed three good responders and four poor responders, but neither t-PA antigen nor t-PAI concentrations showed any clear difference between the two groups. Interestingly, three out of the four patients with a history of thrombosis were good responders. The group with oral ulceration showed no significant difference from the controls for any of these measurements. These findings suggest that the reduced resting fibrinolytic activity is usually due to decreased production of t-PA, but that the defective response to stimulation may be multifactorial in origin. The fibrinolytic abnormalities appear unrelated to the development of thrombosis.