The obstruction of the pulmonary artery or a branch of it leading to the lungs by a blood clot, usually from the leg, or foreign material causing sudden closure of the vessel. (Embolus is from the Greek "embolos" meaning plug.) The risk factors for pulmonary embolism include advanced age, cancer, genetic predisposition, immobilization (especially in the hospital), pelvic or leg trauma, pregnancy, and surgery. The diagnosis of pulmonary embolism can be difficult because the symptoms are nonspecific and may mimic many other diseases. Pulmonary angiography is the gold standard test. Other tests may include oximetry and arterial blood gas analysis and imaging such as chest x-rays and ultrasonography. The treatment includes anticoagulants such as heparin and warfarin (Coumadin). About 10- 15% of patients with pulmonary embolism die.