Life-threatening chronic cor pulmonale occurred in a 22-year-old woman with congenital 3 degrees atrioventricular block, 6 years after implantation of a pacemaker and 3 1/2 years after removal of the pacemaker (the electrodes were too firmly attached to be removed). The emboli originated from the right-atrial thrombi which had formed around the electrodes left in situ. The embolic source shrank during systemic administration of urokinase, initially 600,000 IU in 60 min, then 100,000 IU per hour, and pulmonary perfusion improved transiently. Nonetheless the patient became breathless on the slightest physical exertion. Recurrent syncopal attacks with marked increase of pulmonary artery pressure necessitated pulmonary thrombendarterectomy and removal of the electrode and wires. Thereupon the patient's condition clearly improved so that 6 months postoperatively her exercise tolerance was again unimpaired.