The aim of this retrospective study was to evaluate the efficacy and tolerance of a moderate dose of urokinase (UK : 2000 IU/kg/h) for at least 24 h in a series of 23 patients with massive pulmonary emboli (PE) (mean pulmonary vascular obstruction = 70 +/- 7%). All patients received heparin in conjunction with UK. A control pulmonary angiography, performed at the end of UK therapy, revealed an important decrease in pulmonary obstruction (-50%). One patient died before the end of UK therapy from shock due to PE (early mortality = 4.3%). Severe hemorrhagic complications occurred in 3 patients (13%) and dictated the stoppage of UK administration, but bleeding was never the cause of death or sequelae. Thus, moderate doses of UK associated with heparin appear to effectively induce clot lysis in patients with massive pulmonary emboli. The incidence of hemorrhagic complications should be decreased by strictly respecting all contraindications to the use of thrombolytic agents and by avoiding excessive heparinization.