Prematurity is a recognized risk factor for morbidity and mortality following cardiac surgery. Postoperative and long-term outcomes after cardiac surgery performed in the preterm period are poorly described. The aim of this study was to analyze a population of preterm neonates operated on for critical congenital heart disease (CHD) before 37 weeks of gestational age (wGA) with special attention given to early and late mortality and morbidity. Between 2000 and 2013, 28 preterm neonates (median gestational age (GA) 34.3 weeks) underwent cardiopulmonary bypass (CPB) surgery for critical CHD before 37 wGA; records were retrospectively reviewed. All patients except three with single ventricle physiology had a single-stage anatomic repair. Overall mortality was 43 % (95 % CI 25–62). Risk factors for death were birth weight (p = 0.032) and weight at surgery (p = 0.037), independently of GA, preoperative status, CPB and aortic clamp time. Seven patients, including those with univentricular hearts, died during the postoperative period, and five in the first year after surgery. Median follow-up was 5.9 years (range 1 month–12.8 years). Kaplan–Meier survival rate was 75 % (95 % CI 59–91) at 1 month, and 57 % (95 % CI 39–75) at 1 and 5 years. Eight patients required reoperations after a delay of 2.8 ± 1.3 months; eight had bronchopulmonary dysplasia. At the end of follow-up, nine patients were asymptomatic. One-stage biventricular repair for critical CHD on preterm neonates was feasible. Mortality remained high but acceptable, mainly confined to the first postoperative year and related to small weight. Despite reoperations, long-term clinical status was good in most survivors. Further long-term prospective investigations are necessary to evaluate neurodevelopmental outcomes.