Near-infrared spectroscopy (NIRS) is a noninvasive modality to monitor regional brain oxygenation (rSO2). In this study, we aimed to investigate the correlation between cerebral rSO2 and lactate, pump flow, hematocrit, pCO2, and mean blood pressure (MBP) during cardiopulmonary bypass (CPB). Between March and September 2011, 50 pediatric patients who underwent congenital heart surgery were enrolled into the study. Ages ranged from 6 days to 168 months (median 14 months). A NIRS sensor (Somanetics 5100B, Troy, MI, USA) was placed on the right forehead of patients. CPB period was divided into five stages: 1at the beginning of CBP, 2cooling at 32 degrees C, 3at final hypothermic temperature, 4rewarming at 32 degrees C, 5before weaning from CPB. Data collection included measurements of each parameter at five stages of CPB. Data were analyzed using multivariate analysis within groups and Spearman's correlation to test association between parameters. Lactate levels increased significantly from stage 1 to stage 5 during CPB (P<0.05). There was no significant correlation between cerebral rSO2 and MBPs, pump flows, hematocrit, or pCO2 during CPB. Cerebral rSO2 levels showed changes between the stages; there was a significant increase during cooling period, compared to stage 1 (P<0.05). Significant changes during cooling stage did not happen for other parameters. At stage 3, there was a negative correlation between lactate level and MBP. At stage 4, there was no significant change in cerebral rSO2 levels despite decreased MBP. At the warming stage, low MBPs, but normal rSO2 values, are observed despite increased pump flows. Increased rSO2 levels despite insignificant changes at other parameters during the cooling stage of CPB may show that optimal pump flow with adequate intravascular volume may provide effective cerebral perfusion even without changes in MBP. Considering normal rSO2 values during CPB in this study, it may be speculated that brain protection can be assessed by using NIRS and applying a standard bypass protocol.