In patients with Behçet's disease, venous thrombosis has often been described as a complication. The pathogenesis of this complication, however, has not been fully understood. In this work, various parameters of blood coagulation and fibrinolysis were studied in 20 patients with Behçet's disease and 13 sex-matched healthy volunteers. Patients were classified into three subgroups according to the number of clinical signs involved; group I (no sign): 4 patients; group II (one or two signs): 11 patients; group III (more than three signs): 5 patients. Patients with Behçet's disease, showed an activation of blood coagulation, such as the shortening of prothrombin time (p less than 0.001), decreases in concentrations and activities of plasma antithrombin III (AT-III) (p less than 0.01) and elevated levels of plasma thrombin-antithrombin-III complex (TAT) (p less than 0.01), compared to the control group. Plasma levels (p less than 0.01) and activities (p less than 0.01) of protein C (PC) and total protein S (PS) levels (p less than 0.05) were increased in the patients. Decreased levels of alpha 2-plasmin inhibitor (p less than 0.001) also indicated an activation of fibrinolysis in the patients. When analyzed among the subgroups, patients belong to group II and III showed higher levels of plasma FDP D-dimer (p less than 0.05) and lower levels of plasminogen (p less than 0.05), as compared with patients in group I or control group.(ABSTRACT TRUNCATED AT 250 WORDS)