Blood clots in the large veins of the leg (called deep venous thrombosis) can cause serious problems. If pieces of clot break off, they can travel through the bloodstream to the lungs where they block part of the circulation through the lung, strain the heart, and interfere with allowing oxygen into the blood. Even when clots do not break off, they can damage the leg veins and cause painful leg swelling. Treatment with warfarin (a blood thinner) reduces the risk for these serious complications. Because too much warfarin causes serious bleeding events, it is important to make an accurate diagnosis before treating.