Six infants underwent angiography of the aortic root and the descending aorta with balloon inflation during diagnostic cardiac catheterisation and angiography. In these infants with large proximal left to right shunts, descending aortography with balloon inflation resulted in better opacification of the persistent ductus arteriosus than aortic root angiography, thereby enabling accurate measurement of the persistent ductus arteriosus. Similar systemic and pulmonary vascular impedances had been speculated as the cause of poor or non-opacification of the ductus after ascending aortography. Balloon inflation may have altered the aortic impedance with resulting opacification of the ductus. Descending aortic angiography with balloon inflation is also useful in opacifying the isolated ductus and in demonstrating aortopulmonary collaterals in patients with pulmonary atresia.