To compare the effects of heparin thrombolytic agents in pulmonary thromboembolic disease, we randomly assigned 40 patients with pulmonary emboli but without other clinical cardiopulmonary disease either to heparin followed by oral anticoagulants (21 patients) or to urokinse or streptokinase followed by heparin and then by oral anticoagulants (19 patients). The effects on pulmonary-capillary blood volume and diffusing capacity were compared at two weeks and at one year. The pulmonary-capillary blood volume (in milliliters per square meter of body-surface area) was abnormally low (30 +/- 2.4) [+/- S.E.]; normal, 47 +/- 1.5) in the heparin-treated group at two weeks and remained unchanged at one year. In contrast, it was normal (45 +/- 2.5) in the group receiving thrombolytic agents, both at two weeks and at one year (P < 0.001). The pulmonary diffusing capacity was reduced to 69% of the predicted value in the heparin group at two weeks and 72% at one year, whereas it was 85% of the predicted value in the thrombolytic group at two weeks and 93% at one year (P < 0.001). These results indicate that thrombolytic agents allow more complete resolution of thromboemboli than do heparin and anticoagulants and that they improve capillary perfusion and diffusion.