Deep venous thrombosis (DVT) is a blood clot in a large vein, usually in the leg or pelvis. Sometimes a DVT detaches from the site of formation and becomes mobile in the blood stream. If the circulating clot moves through the heart to the lungs it can block an artery supplying blood to the lungs. This condition is called pulmonary embolism. The disease process that includes DVT and/or pulmonary embolism is called venous thromboembolism (VTE). Each year in the United States, an estimated 350,000-900,000 persons develop incident VTE, of whom approximately 100,000 die, mostly as sudden deaths, the cause of which often goes unrecognized. In addition, 30%-50% of persons with lower-extremity DVT develop postthrombotic syndrome (a long-term complication that causes swelling, pain, discoloration, and, in severe cases, ulcers in the affected limb). Finally, 10%-30% of persons who survive the first occurrence of VTE develop another VTE within 5 years.