Deep vein thrombosis and pulmonary embolism are important clinical disorders that affect patients in many medical and surgical specialties. In North America, deep vein thrombosis results in hospitalization of up to 600,000patients per year. Pulmonary embolism is also a serious problem that occurs in more than 500,000patients per year, of whom approximately 200,000will die. Almost half of those patients who die will be terminally ill or suffer an incurable disease, but the remainder of the deaths occur in patients who other- wise could have recovered completely.'