Hyperlactatemia and unmeasured anions (UMA) have been suggested to be useful predictors of outcomes after pediatric cardiac surgery in the ICU. However, if we detect high-risk patient in the operating room, we could practice early intervention to decrease mortality and morbidity. The purpose of this study was to determine whether the intraoperative lactate or UMA levels can predict adverse outcomes in pediatric cardiac patients with undergoing cardiopulmonary bypass (CPB). We studied 102 patients with congenital heart disease. Arterial blood samples were obtained after inducing anesthesia, 5 min after weaning from CPB and after chest closure. Major adverse events (MAEs) were defined as cardiac compression, re-sternotomy due to hemodynamic instability, extra-corporeal membrane oxygenator support, creatinine levels greater than 2 mg/dL, or death. Patients were divided into MAE group (8 patients, 7.8%) and non-MAE group. Six patients with MAEs died. Importantly, the lactate levels (mmol/L) at weaning from CPB (4.19 vs 2.1; MAE group vs non-MAE group), chest closure (5.76 vs 2.39; MAE group vs non-MAE group) and the intraoperative increases in lactate levels were significantly higher in the MAE group than in the non-MAE group. However, there was no significant difference in the UMA levels or their changes between the groups. The increase in the lactate level from CPB weaning to chest closure was the best predictor of MAEs (AUC: 0.810). In conclusion, the intraoperative plasma lactate levels were more closely associated with MAEs, and they are more useful for predicting the outcome of pediatric cardiac patients than the UMA levels.