Neurodevelopmental impairment due to delayed brain development and brain injury is a fundamental problem in children with critical congenital heart disease (CCHD). Significant longterm motor-, cognitive-, and behavioral problems are the result of early postnatally and perioperatively induced brain injury. Allopurinol, a xanthine oxidase inhibitor, prevents the formation of toxic free oxygen radicals, thereby limiting hypoxia-reperfusion damage. Both animal and neonatal studies suggest that administration of allopurinol reduces hypoxic-ischemic brain injury, is cardioprotective, and safe. This study aims to evaluate the efficacy and safety of allopurinol administered early postnatally and perioperatively in children with a CCHD requiring cardiac surgery with cardiopulmonary bypass.