The descending aorta appears small relative to unusually large pulmonary arteries in infants with atrioventricular septal defects. To determine whether this discrepancy in great arterial size influences the outcome after repair of atrioventricular septal defects, we reviewed our experience with 34 infants younger than one-year-old undergoing primary repair. There were four early deaths (11.8%), one in 16 patients with normal chromosomes (6.3%), and three of 18 with Down's syndrome (16.7%). There was no difference between survivors and non-survivors in terms of either the size of the aorta at the level of the diaphragm (descending aortic index, 149±34 versus 139±46 mm2/m2), the pulmonary arterial index (675±170 versus 745±182 mm2/m2), or the ratio of the descending aorta to the pulmonary trunk arterial (4.85±1.6 versus 5.93±2.8). Compared with survivors, non-survivors had higher ratios of vascular resistance (0.48±0.42 versus 0.23±0.14, p<0.05), and tended to have larger preoperative cardiothoracic ratios (0.63±0.05 versus 0.55±0.08, p=0.12) Compared to patients with normal chromosomes, the patients with Down's syndrome tended to have smaller descending aortas (140±34 versus 159±34 mm2/m2, p=0.12), higher ratios of vascular resistance (0.32±0.2 versus 0.18±0.1, p=0.08), and higher mean pulmonary arterial pressures (47±11 versus 39±18 mm Hg, p=0.12), suggesting that pulmonary vascular obstructive disease may be more severe in patients with Down's syndrome. The size of the descending aorta and pulmonary arteries do not influence outcome after repair of atrioventricular septal defect in infants.