A comparison of the effects of two regimens of urokinase infusion not associated to heparin and of continuous heparin infusion was made assessing the rate of emboli and arterial hypoxemia resolution in patients with acute pulmonary embolism. Twenty-nine patients with acute pulmonary embolism, each one diagnosed by means of perfusion lung scan and selective pulmonary arteriography, were admitted to the study and randomly allocated to three treatment groups: A, 10 patients, urokinase: 800,000 CTA units/day for three days followed by oral anticoagulants; B, 9 patients, heparin: mean daily dose 30,000 units for seven days followed by oral anticoagulants; C, 10 patients, urokinase: 3,300,000 CTA units in 12 hours followed by oral anticoagulants. Pulmonary lung scan and PaO2st (PaO2 standardized to PaCO2 of 40 mmHg) were repeated at 1, 3, 7, and 30 days in all the groups of treatment. The mean daily rate of improvement, assessed from both the perfused lung segments and the PaO2st increment, in group C was highest in the first 24 hours and lowest from the first to the third day of treatment. However, from the third to the seventh day of treatment and onwards the mean daily rate of improvement was roughly the same in all the groups. After one month, lung perfusion and the PaO2st had considerably improved, but had not attained full recovery in any of the treatment groups.(ABSTRACT TRUNCATED AT 250 WORDS)