Sixteen patients who had massive pulmonary thromboembolism and shock had no history of cardiopulmonary disease. We present an evaluation of the short-term effects of fibrinolytic treatment consisting of intrapulmonary administration of a bolus of 500,000 IU of urokinase followed by infusion of 1 x 10(6) IU into the right auricle over 12 h and subsequent intravenous infusion of heparin. For each patient, the effectiveness of treatment was evaluated by comparing pretreatment angiographic and hemodynamic parameters with those measured 48 h after the start of treatment. The Miller index fell from 22.9 +/- 5.9 to 9.8 +/- 3.3 (p less than 0.001), with a mean improvement of 57.2 percent. All the hemodynamic parameters studied (cardiac output and index, total pulmonary vascular resistance, and systolic, diastolic, and mean pulmonary vascular pressure) also exhibited statistically significant differences between pretreatment and posttreatment values (p less than 0.001 for each parameter), with a mean improvement of over 30 percent in each case. All the patients survived, and in no case did treatment fail; only one patient (6.2 percent) suffered severe hemorrhage. We conclude that this form of administration of urokinase is useful for patients with critical massive pulmonary thromboembolism.