The greater saphenous vein is a large superficial leg vein running from the foot to the groin, roughly in parallel with the short or lesser saphenous vein, which runs up the back of the leg from the ankle to the knee. These are the principal vessels of the superficial venous system in the leg. The greater saphenous vein delivers oxygen-depleted blood from the ankle, lower leg and thigh vessels to the femoral vein, the primary deep vein in the leg. The propulsion of blood up such a significant length of the body is heavily dependent on the function of the one-way valves that prevent backflow down the vein. When those valves fail, the resultant venous reflux causes blood to back up into the smaller veins closer to the skin, distending them and initiating the condition known as varicose veins. Venous insufficiency, also known medically as venous reflux disease, in the greater saphenous vein is a primary underlying cause of varicose veins. The great saphenous is also therefore the superficial vein most frequently treated with radiofrequency ablation to restore healthy circulation and diminish varicose veins.