Older statistics estimate that in the United States, acute pulmonary thromboembolism afflicts 500000 to 600000 persons annually and is either a primary or secondary cause of death in 150 000 to 200 000 of these individuals. 1, 2 Extrapolation of a population-based study from data accumulated in 1985 through 1986 in Worcester, Mass, suggests that each year there are 170 000 new cases of clinically recognized venous thromboembolism treated in short-stay hospitals and 99000 hospitalizations for recurrent disease. 3 When the disease process was followed from the time of clinical recognition, the 1-year mortality rate in a national multicenter project (PIOPED) was reported as 25%, with 2.5% dying from pulmonary embolism itself and most patients dying from the major diseases that are associated with pulmonary thromboembolism, including cancer, various infections, cardiovascular diseases, and other pulmonary diseases. 4 Other studies have reported that in patients without preexisting cardiac or pulmonary disease, the 1-year mortality rate ranged from 3% to 9%. 5, 6 If a massive pulmonary embolism occurs associated with systemic hypotension, the in-hospital mortality rate is 18%. 7 The persistence of pulmonary hypertension after embolization has been associated with increased mortality rates; the higher the pulmonary artery pressure, the lower the survival rate at 5 years. 8 Furthermore, it has been predicted, with admitted imprecision, that 0.01% of patients develop what is now commonly referred to as chronic pulmonary thromboembolic pulmonary hypertension. 9