Objective: To review, in retrospective fashion, the effect of preoperative mechanical ventilation on neonatal outcomes after cardiac surgery. Methods: We studied 114 newborns less than 15 days old admitted to the cardiac intensive care unit for cardiac surgery. Of the newborns, 71 (62%) were mechanically ventilated at the referring hospital before transport to our institution. Of the 71 ventilated patients, 14 were extubated and breathing spontaneously before cardiac surgery. We compared variable haemodynamics and outcomes between the 57 patients mechanically ventilated at time of cardiac surgery, and the 57 patients breathing spontaneously at this time. Results: Newborns mechanically ventilated before cardiac surgery had increased preoperative haemodynamic compromise, increased postoperative sepsis (p equal to 0.02) and mortality (p equal to 0.005) compared with those breathing spontaneously before cardiac surgery. Conclusion: Newborns requiring preoperative mechanical ventilation had greater risk of postoperative morbidity and mortality. Heightened vigilance is warranted in this population of patients at high risk.