We describe an infant born prematurely at 30 weeks gestation with Down's syndrome who became dependent on oxygen at 3 weeks of age after an uneventful initial neonatal period. There had been no evidence of bronchopulmonary dysplasia. An isolated aortopulmonary collateral artery of moderate size was mistakenly diagnosed as persistent patency of the arterial duct on echocardiography, and subsequently successfully occluded using two coils with an excellent clinical result. To the best of our knowledge, this is the first description of a congenital aortopulmonary collateral artery in a symptomatic infant with Down's syndrome and no evidence of bronchopulmonary dysplasia. We discuss the possible etiologies of these collateral arteries. It is important to include aortopulmonary collateral arteries in the differential echocardiographic diagnosis of an arterial duct.