Transcatheter embolization of 25 aortopulmonary collateral arteries (7 bronchial arteries and 18 intercostal arteries) was attempted prior to intracardiac repair in 7 patients. The underlying disease was tetralogy of Fallot in 3 patients, pulmonary atresia with ventricular septal defect in 2, double-outlet right ventricle with ventricular septal defect and pulmonary stenosis in 1 and tricuspid stenosis with pulmonary atresia in 1. The intervals between embolization and intracardiac repair ranged from 0 to 17 days (mean 4.5 days). Embolization resulted in total occlusion in 7 bronchial arteries and 17 intercostal arteries, with an overall success rate of 96%. Complications included a coil dislodgement from a collateral artery into the aorta in one patient, necessitating surgical removal of the dislodged coil from the femoral artery, an exacerbation of cyanosis and dyspnea on exercise in 5, and slight fever in 2. In one patient with tetralogy of Fallot, who had 5 collateral vessels, transcatheter embolization caused hypoxemia, bradycardia and hypotension and therefore intracardiac repair was performed immediately after embolization. Aortopulmonary collateral arteries in patients with congenital heart disease can be effectively treated by transcatheter embolization. Embolization should be performed just before intracardiac repair because an excessive decrease in arterial oxygen saturation after embolization may require an emergency operation.