Background: Conflicting data exist regarding the impact of ascending aorta size on outcomes after the Norwood procedure. Results from multi-institutional studies have largely relied on heterogeneous populations undergoing this surgery for different anatomic defects. Using data from the Single Ventricle Reconstruction Trial, we analyzed the impact of preoperative ascending aortic diameter on Norwood outcomes for patients with aortic atresia variants of hypoplastic left heart syndrome. Methods: Neonates with aortic atresia and no ventricular septal defect were included and classified into four groups, based on their baseline ascending aorta echocardiographic measurements: less than or equal to 1.5 mm, 1.6 to 1.9 mm, 2.0 to 3.9 mm, and greater than or equal to 4.0 mm. Outcomes included 14-day mortality, transplant-free survival at 1 and 14 months, need for extracorporeal membrane oxygenation, length of ventilation, intensive care, and hospital stay, intensive care unit (ICU)-free days, right ventricular function, and incidence of recoarctation by 14 months. Results: Overall, 292 patients were analyzed. Median length of ICU stay was significantly longer for infants with small aortas, and ICU-free days were significantly lower. There was no difference in length of mechanical ventilation or hospitalization between groups. Long-term right ventricular function and tricuspid regurgitation did not differ. Aortic arch recoarctation incidence was higher in patients with small aortic diameters. Patients with aortas less than or equal to 1.5 mm had decreased 30-day transplant-free survival. Conclusions: Infants with aortic atresia variants of hypoplastic left heart syndrome and baseline ascending aortic diameter less than or equal to 1.5 mm appear to suffer the greatest morbidity and mortality early after Norwood procedure. These infants also experienced longer stays in the ICU and higher rates of recoarctation. Ascending aortic diameter does not appear to affect long-term ventricular function.