Deep venous thrombosis and pulmonary embolism are significant causes of morbidity and mortality in the United States; estimates range from 120,000 to 150,000 deaths annually. Although usually symptomatic, deep venous thrombosis can be clinically occult, in part due to incomplete obstruction or in part related to duplication, triplication, and fenestration anomalies, primarily of the superficial femoral or popliteal vein. Additionally, pulmonary emboli caused by deep venous thrombosis may be clinically silent. Because of therapeutic implications, especially indications for insertion of inferior vena caval filters, comprehensive assessments of both the disease process (i.e., deep venous thrombosis) and the complication (i.e., pulmonary emboli) are important. Thus, when a pulmonary embolus is the presenting process, correlative assessment of deep venous thrombosis, even in the absence of symptoms or signs in the lower extremity, may be of therapeutic significance. Conversely, when deep venous thrombosis of the lower extremities involving the popliteal or superficial femoral vein is the presenting process, correlative assessment of the pulmonary circulation, even when no pulmonary symptoms or signs are present, may be of therapeutic significance. Relative to the diagnosis of pulmonary embolism, the roles of assays of D-dimer, ventilation-perfusion lung scans, and segmental occlusion studies of the pulmonary circulation are discussed. Finally, the indications for insertion of inferior vena caval filters above the renal veins are presented and examples are shown.