In a neonate born prior to term with a weight of 1825 grams, and diagnosed prenatally as having atrioventricular septal defect and Down's syndrome, we found the aortic arch to be interrupted between the left carotid artery and the left subclavian artery, with the arterial duct being the only route of distal perfusion. Three days later, however, echocardiographic interrogation revealed marked collateral connections between the aortic arch and the descending aorta, the picture then mimicking coarctation rather than interruption of the aortic arch. The rapid development of the collateral arteries was confirmed by magnetic resonance imaging and during cardiac surgery.