Deep venous thrombosis (DVT) is clotting of blood in a deep vein of an extremity (usually calf or thigh) or the pelvis. DVT is the primary cause of pulmonary embolism. DVT results from conditions that impair venous return, lead to endothelial injury or dysfunction, or cause hypercoagulability. DVT may be asymptomatic or cause pain and swelling in an extremity. Diagnosis is by history, physical examination, and duplex ultrasonography, with D-dimer or other testing as necessary. Treatment is with anticoagulants. Prognosis is generally good with prompt, adequate treatment; common long-term complications include venous insufficiency with or without postphlebitic syndrome.