OBJECTIVES To study the long-term neurodevelopmental effects of hyperglycemia in infant bypass surgery for congenital heart disease (CHD). STUDY DESIGN Prospective cohort study on neurodevelopmental outcome after infant cardiac bypass surgery. EXCLUSION CRITERIA age older than 1 year at first surgery, genetic comorbidity, and birth weight <2000 g. Of 167 eligible infants, follow-up examination at 4 years was completed in 150 children (90%). Intraoperative and postoperative highest and lowest glucose levels within 24 hours after bypass surgery were prospectively collected. Neurodevelopmental outcome at 4 years of age was assessed using standardized IQ tests and the Movement Assessment Battery for Children-second version for motor outcome assessment. RESULTS Mean age at surgery was 2.8 months (0.1-10.7 months), 33% of children had an acyanotic CHD and 67% a cyanotic CHD. Glucose levels were elevated (>8 mmol/L) in 21 (14%) children in the first 24 postoperative hours. Glucose levels normalized in all children within 48 hours, 7 children (4%) received insulin infusions. Mean total IQ was within the normal range, but significantly lower than the normal population (92.5 [SD 15.0], P < .001). Higher postoperative glucose levels were related to better cognitive performance in the univariate analysis (P < .03), but not when other risk factors were taken into account. Independent risk factors for adverse outcome were lower socioeconomic status, higher risk adjustment for congenital heart surgery score, and longer duration of intensive care stay. CONCLUSION Hyperglycemia is common in the early postoperative course after infant bypass surgery for CHD and normalizes within 48 hours. Hyperglycemia has no adverse effect on long-term neurodevelopmental outcome.