A LL PATIENTS with clinically recognized pulmonary embolism have the constant threat of a recurrent episode which may prove fatal, making it imperative that treatment be give, n to prevent this development. In fact, autopsy studies indicate that in more than two-thirds of cases with fatal pulmonary embolism, evidence is found of prior emboli having occurred in the days or weeks preceding the fatal episode, and it has been suggested that untreated, more than one-third of patients will have recurrence. 1-3 Contrariwise, if recurrence is prevented, significant resolution of the pulmonary obstruction occurs over a period of weeks or months which returns the pulmonary circulation to preembolic condition in most cases. 4 Thus, in the majority of patients with pulmonary embolism, the primary objective of therapy is to prevent recurrence, and therefore, a course of anticoagulants is the only therapy indicated, and outlook for recovery is excellent. In a few cases, either because of the size or number of emboli, or presence of preexisting abnormality of the pulmonary circulation, there is severe compromises of cardiopulmonary dynamics which threaten survival. In such patients, it is desirable to accelerate removal of the obstruction of the pulmonary circulation as well as to prevent recurrence, and in those cases thrombolytic therapy may be of value.