Background: The monitoring of regional cerebral O2 saturation (rSO2) with near-infrared spectroscopy (NIRS) is a noninvasive technique to measure tissue oxygenation in the brain. It may be an effective monitoring technique in the early diagnosis of pre-, intra- and post-operative insufficient oxygen supply to the brain in surgery for congenital heart diseases. In pediatric patients, a variety of clinical and laboratory parameters, including serum lactate and regional cerebral oxygen saturation, may be helpful in monitoring global tissue and cerebral oxygen delivery and consumption. Aim: Our study was designed to investigate whether there is a correlation between the NIRS scores and serum lactate levels during congenital heart surgery. Our secondary objective was to define the predictive value of this correlation on the duration of extubation and intensive care unit stay. Method: A total of 82 successive neonatal and infant patients with complex cardiac pathologies were enrolled in the study. Blood lactate levels and NIRS values were measured during the phases of anesthesia induction, sternotomy, cannulation, onset of CPB, the beginning of aortic cross-clamping and the end of the CPB. Study design: Prospective randomized Results: Patients with normal rSO2 /normal lactate during the operation represented the largest percentage of patients during anesthesia induction (n=50, 60.9%) and sternotomy (n=54, 65.8%). The only negative correlation between lactate and rSO2 was detected during anesthesia induction. The time to extubation and the stay in the intensive care unit were longer in patients with low rSO2 values during anesthesia induction and sternotomy. In the same periods, elevated lactate levels were associated with longer time to extubation and intensive care unit stay than the patients with normal lactate levels. Conclusion: In our study, no correlation was demonstrated between NIRS scores and serum lactate levels in children during congenital heart surgery, except for anesthesia induction. The only negative correlation between the decrease in NIRS scores and the increase in serum lactate levels was observed during anesthesia induction. This result indicated that cerebral NIRS monitoring cannot be used as an indicator of global hypoperfusion in the same way as lactate.