Skin blood flow in the lower leg is increased by external compression in standing subjects. This prevents the hypoxia induced by standing. The maximum increase in oxygen tension is seen when the external pressure is 40-50 mmHg. Pressures in the range of those produced by lightweight elastic stockings were found to be ineffective. Our results uphold the commonly held clinical opinion that only compression bandages or strong stockings are effective in the treatment of venous ulcers. External compression when the subject is recumbent causes skin oxygen tension to fall. Uniform external pressure exceeding 10 mmHg should therefore be avoided in the prophylactic treatment of post-operative deep-vein thrombosis. This observation suggests that when the patient is confined to bed, only lightweight stockings are safe in the prophylaxis of post-operative deep-vein thrombosis.