44 patients were treated with subcutaneous ancrod (Arwin) for peripheral arterial occlusive disease (Fontaine stage III). An acute fulminating arterial occlusion occurred in two patients on the 10th and 23rd days respectively, and in a third patient a deep venous thrombosis developed in the leg on the 45th day after beginning treatment. Accordingly, vascular occlusion must be expected during defibrination in the presence of poor peripheral drainage conditions and if the serum fibrinogen can no longer be maintained below 1.0 g/1 in spite of increasing the ancrod dosage. Under these circumstances the treatment must be terminated.