BACKGROUND Compression therapy needs to narrow the veins of patients with venous disorders to achieve a hemodynamic effect. This study investigated the external pressure necessary to narrow and occlude leg veins in different body positions. METHODS In nine healthy volunteers and five patients with incompetent small saphenous veins, the diameter of the small saphenous veins and of one posterior tibial vein was measured at the mid level of the calf by duplex ultrasound scans in sitting, standing, and supine positions. A modified blood pressure cuff with an acetate window that permitted ultrasound visualization of the veins was gradually inflated, and the pressures needed to narrow or occlude the veins were recorded. RESULTS Initial narrowing occurs with a median pressure of between 30 and 40 mm Hg in the sitting and standing positions. Complete occlusion of superficial and deep leg veins occurs with 20 to 25 mm Hg in the supine position, between 50 and 60 mm Hg in the sitting position, and at about 70 mm Hg in the standing position. The difference between sitting and standing, and between standing and supine, was statistically significant (P < .01 and P < .001, respectively). CONCLUSIONS The external pressure has to exceed the hydrostatic pressure in the vein to compress leg veins effectively. Higher external pressures than can be expected to be delivered by elastic compression stockings are required to achieve the collapse of lower-extremity veins in the upright position.