Background: Hyperglycemia during cardiopulmonary bypass may increase the incidence and severity of neurologic deficits that may result from cerebral ischemia. Moderate hyperglycemia has been noted to occur in pediatric patients undergoing cardiac surgery despite measures such as eliminating dextrose from the CPB clear pump priming solution and from the intra-operative iv fluids. To ameliorate the hormonal and hemodynamic stress responses during cardiac surgery in neonates, infants and children, high dose fentanyl anesthesia is widely used. The authors wished to determine prospectively whether fentanyl dosage is associated with reduced blood glucose or not in pediatric patients undergoing cardiac surgery. METHODS: Twenty four pediatric patients undergoing cardiac surgery were allocated randomly into 3 groups who received 25 g/kg fentanyl, 50 g/kg fentanyl or 75 g/kg fentanyl before CPB. The changes of plasma glucose and insulin levels were observed after sternotomy, on bypass, 30 min after bypass, off bypass, and the end of the operation. RESULTS: Blood glucose levels were not increased after sternotomy, but significantly increased at bypass to the end of the operation in all fentanyl dosage groups. Plasma insulin level increased, but statistically not significant. Different fentanyl dosage (25~75 g/kg) is not associated with differences in blood glucose level. CONCLUSION: At a dosage of 25~75 g/kg fentanyl anesthesia during pediatric open heart surgery were associated with no differences and below 250 mg/dl of blood glucose level and no significant changes in insulin level.