Acute pulmonary embolism is one of the most serious of the life-threatening illnesses, but it can be prevented or effectively treated. Unfortunately, there are no safe, rapid, or inexpensive tests for its detection, and so a high degree of alertness remains the clinician's best ally in the diagnosis of this disorder. Among the more useful diagnostic measures, a radioisotope ventilation-perfusion lung scan may be done in the search for a mismatch-an area normally ventilated where there is a perfusion defect. If two or more segmental mismatches are found, there is a high probability that pulmonary embolism is present. A normal perfusion scan virtually eliminates the possibility of pulmonary embolism, but an abnormal ventilation-perfusion scan is not synonymous with the diagnosis of pulmonary embolism and may lead to the overdiagnosis of this disorder. Subcutaneous administration of heparin has proved to be an effective measure in preventing deep venous thrombosis, but it must be started before any thrombosis occurs. Intravenous use of heparin and oral anticoagulation are the mainstays of the therapy of deep venous thrombosis and pulmonary embolism.