Ulcers are open skin sores. They can affect any area of the skin. But they most often occur on the legs. Venous ulcers are leg ulcers caused by problems with blood flow (circulation) in your leg veins. Normally, when you get a cut or scrape, your body's healing process starts working to close the wound. In time, the wound heals. But ulcers may not heal without proper treatment. Causes of venous ulcers Venous ulcers most often form around the ankles. Venous ulcers typically occur because of damage to the valves inside the leg veins. These valves control the blood pressure inside the veins. They allow it to drop when you walk. If the blood pressure inside your leg veins doesn't fall as you're walking, the condition is called sustained venous hypertension. That increase in blood pressure causes ulcers to form on your ankles. Venous ulcers may also be caused by other problems with your leg veins. These include: Varicose veins. These are large, bulging leg veins. They occur when valves in the leg veins don’t work well, allowing blood to collect (pool) in the lower leg. Chronic venous insufficiency. Like varicose veins, this condition occurs when your leg veins can’t pump blood back up to your heart. Blood then pools in your lower legs, causing your legs to swell. Since the blood can't flow well in your legs, the swelling may be extreme. This extreme swelling can put so much pressure on your skin that venous ulcers form.