Embolization of major aortopulmonary collateral arteries by coils introduced via catheterization is a well-established procedure for the control of excessively high pulmonary blood flow in patients with tetralogy of Fallot and pulmonary stenosis or atresia. Under special circumstances, this procedure can be life-saving. Between October 1987 and February 1992, four patients with tetralogy of Fallot and pulmonary atresia and one patient with pulmonary stenosis rather than atresia underwent transcatheter embolization of major aortopulmonary collateral arteries as an emergency. Their ages ranged from 41 days to 13 years, with a mean of 45 months. Two patients had recently undergone surgical repair of tetralogy of Fallot while one had been palliated by enlarging the right ventricular outflow tract without closing the ventricular septal defect. All three had been dependent on mechanical ventilation in the postoperative period with signs of pulmonary congestion or intraparenchymal hemorrhage. Eight embolizations were attempted with 0.025 or 0.038 inch coils; seven collateral arteries were completely occluded and one was partially occluded. Thirteen coils were used in total. The systemic saturation rose in one postsurgical patient, with radiological evidence of decreased pulmonary congestion and regression of hemorrhage. In the other cases, the systemic saturation was unchanged. Peak pulmonary arterial pressures dropped in two patients but remained unchanged in three. Our experience suggests that embolization of major aortopulmonary collateral arteries can be beneficial in critical patients, thus avoiding the risks of surgical reexploration.