Although some evidence suggests benefit of steroid supplementation after pediatric cardiac surgery, data correlating adrenal function with the postoperative course is scarce. This study sought to determine if adrenal insufficiency (AI) after cardiac surgery is associated with a more complicated postoperative course in children. A prospective study was performed during a 6-month period at a pediatric medical center. Included were 119 children, 3 months and older, who underwent heart surgery with cardiopulmonary bypass. Cortisol levels were measured before and 18 h after surgery. Patients were divided into two groups by procedure complexity (low or high), and clinical and laboratory parameters were compared between patients with and without AI within each complexity group. In the low-complexity group, 45 of the 65 patients had AI. The normal adrenal function (NAF) subgroup had greater inotropic support at 12, 24, and 36 h after surgery and a higher lactate level at 12 and 24 h after surgery. There were no significant differences between subgroups in duration of ventilation, sedation, intensive care unit (ICU) stay, or urine output. In the high-complexity group, 27 patients had AI, and 27 did not. There were no significant differences between subgroups in inotropic support or urine output during the first 36 h or in mechanical ventilation, sedation, or ICU stay duration. Children with AI after heart surgery do not have a more complex postoperative course than children with NAF. The adrenal response of individual patients seems to be appropriate for their cardiovascular status.