A new radiological technique is described for opacification of the thoracic aorta in neonates and babies weighing less than 10 kg. The exploration method is described: injection of a contrast medium after retrograde carotid puncture, and increased endothoracic pressure. The results obtained in 21 babies aged from 3 to 157 weeks, and weighing 3.3 to 10 kg are discussed. Radiographic results were excellent in 17 cases, satisfactory in 3 cases, and only moderate in 1 case. The procedure was perfectly well tolerated, and enabled visualization of an arterial canal in 4 cases, stenosis of a Blalock-Taussig anastomosis in 1 case, aortic stenosis in 7 cases (2 tubular hypoptasias, 1 coarctation, 3 segmentary interruptions, 1 recoarctation), coronary-cardiac fistula in 1 case, and a normal aorta in 9 cases. The authors review the advantages, inconveniences, and risks of the other methods of opacification of the thoracic aorta in neonates and babies. They conclude that when right cardiac catheterization cannot give, in babies under 10 kg of weight, a satisfactory radiological assessment of the thoracic aorta in the following clinical conditions: coarctation syndrome, badly tolerated left-right shunt, or possible abnormality of the aortic arch or coronaries, the technique of opacification of the thoracic aorta by retrograde puncture of the common carotid is the best radiological method for establishing a definite diagnosis.