Although the exact cause of DVT is not known, venous thrombosis and its sequelae remain important clinical problems. Pulmonary embolism is a significant cause of morbidity and mortality in the hospitalized population, and the postthrombotic syndrome affects a large portion of the general population. While specific screening tests are not readily available to detect those patients who are likely to develop DVT, certain clinical risk factors have been identified that predispose to thrombosis. These groups include patients undergoing a wide variety of surgical procedures, patients with cardiac disease or cancer, pregnant or postpartum women, and individuals with previous history of DVT. The diagnosis of thrombosis is based on clinical findings and must be confirmed with appropriate laboratory tests. While contrast venography remains the gold standard, noninvasive tests have become increasingly more accurate. The recent use of real-time B-mode ultrasonic imaging and duplex sonography for the diagnoses of DVT has been shown to be efficacious. The postthrombotic syndrome with its associated chronic pain and ulcerations remains a significant clinical problem. The general diagnosis of this condition is readily made on clinical grounds in the advanced state. However, exact knowledge of the location and cause of the venous pathology can only be obtained using objective diagnostic tests. Older noninvasive and invasive tests may diagnose the presence of venous obstruction, valvular incompetence, and also may document venous hypertension.(ABSTRACT TRUNCATED AT 250 WORDS)