In arterial thrombectomy, a powerful current of blood indicates that all obstacles in the flowpath have been removed successfully. If, however, venous thrombectomy is performed, backflow from more proximal segments occurs only in the absence of competent venous valves at sites nearer to the heart. According to Basmajian (1952), valves are absent above the sapheno-femoral junction in 20% of the veins. According to Di Dio (1949), a valve is found in the common iliac vein in 7%; but most of these valves are small and incompetent. In the external iliac vein, valves are found in approximately 25%. According to Basmajian, more than 2/3 of these valves are competent.