THE national impact of thrombosis should not be underestimated. Arterial thromboembolism is a major, if not the primary, contributor to the number 1 (acute myocardial infarction), number 3 (stroke), and the number 4 (renal) causes of death in the United States. In addition, thromboembolic episodes are frequently associated (either causally or as a complication) with rheumatic or atherosclerotic valvular diseases, arrythmias, congestive heart failure, and intimal damage in the carotid artery, the aorta-iliac region, and the arteries of the lower extremities. Some infections and severe blood dyscrasias also may be associated with arterial thrombosis. Venous thromboembolism causes approximately 300,000 patients annually to be hospitalized in the United States, of which more than 50,000 die. The cost of the hospitalization of the 250,000 survivors has been calculated to be approximately $750 million per year. These figures do not include the morbidity and cost for patients with venous thrombosis alone, nor the