A retrospective study of 177 patients (137 male, 40 female) with a disease that fulfilled the criteria of complete or incomplete Behçet's disease showed 106 thrombotic lesions of a venous trunk in 62 (35%) of these patients, superficial phlebitis and retinal vein thrombosis excluded. The incidence of thrombosis was unrelated to race and sex. The venous lesions were single in 38 cases and multiple in 24 cases, including 4 with more than 4 territories involved. Arterial lesions were also present in 7 cases. The lesions were located in the sural (n = 63), iliofemoral (n = 14), inferior vena cava (n = 13), superior vena cava (n = 3), axillary (n = 2) and cerebral (n = 13) veins or territories. Thrombosis developed during the first 4 years of Behçet's disease in one half of the patients. It preceded the aphthosis by 1 to 10 years in 11 cases, was concomitant with it in 15 cases and appeared more than 10 years after the diagnosis in 8 cases. Pulmonary embolism occurred in 11 patients, requiring clipping of the vena cava in 2, but it was never lethal. However, 2 of the 5 patients who died had venous thrombosis (Hughes Stovin syndrome). Thirty-eight patients could be followed up for 12 to 192 months (mean 43) under anticoagulant or antiplatelet therapy. Five vascular recurrences (13%) were observed, 2 of them (1 arterial graft thrombosis, 1 cerebral thrombosis) under anti-vitamin K agents. Cerebral thrombosis (9 men, 4 women) accounted for 27% of the neurological lesions; its prognosis was favourable in the 7 patients successfully treated with anticoagulants. Thrombosis could not be explained. Haemostasis studies showed a decrease of fibrinolytic activity compared to a group of normal subjects and an increase in fibrinogen, factor VIII and Willebrand's factor levels; these abnormalities did not seem to be specific. Thus, phlebitis truly is an element of Behçet's disease. Until its mechanism is better understood, effective anticoagulant therapy in cases with proven thrombosis and preventive antiplatelet therapy in the other cases seem to be rational measures.