This study aimed to examine the association between lactate levels in the first hours after surgery for congenital heart defects and the results of Risk-Adjusted Classification for Congenital Heart Surgery (RACHS-1) scoring and to evaluate serial lactate levels over time to determine whether they can serve as a supplementary tool for postoperative assessment within the same RACHS-1 group of patients. A retrospective cohort study was performed using data retrieved from a clinical database of 255 children who had surgery for congenital heart defects between 1999 and 2001 at Sheba Medical Center. Lactate levels were measured postoperatively four times (mg/dL units). The last sample was taken at the end of the surgical procedure, and lactate levels were measured at admission to the pediatrics critical care unit, then 6 and 12 h after admission. The lactate level was measured via arterial blood gases. A total of 27 deaths occurred, yielding a mortality rate of 7.4% when Norwood operations were excluded and 10.16% when they were included. The mean initial postoperative lactate level was significantly lower for survivors (42.2 ± 32.0 mg/dL) than for nonsurvivors (85.4 ± 54.1 mg/dL) (p < 0.01). The serial mean lactate levels decreased progressively for all surviving patients (r (2) = 0.96) compared with nonsurvivors (r (2) = 0.02). The lactate levels correlated with the RACHS-1 subgroups at each time point (r (2) > 0.96 for all). The Pearson correlations between postoperative lactate levels (last lactate measurement taken in the operating room) and cardiopulmonary bypass (CPB) duration (r = 0.549), clamp duration (r = 0.586), and the inotropic score (r = 0.466) (p < 0.001 for all) were significantly positive. The correlations between the maximum lactate levels (during the first 12 postoperative hours) and CPB duration (r = 0.496), clamp duration (r = 0.509), and the inotropic score (r = 0.633) (p < 0.001 for all) were extremely positive. The early elevation of lactate levels in RACHS-1 subgroups 1 to 3 were highly correlated with poor prognosis and death (p < 0.03). In addition, the lactate levels differed significantly between survivors and nonsurvivors within the same RACHS-1 subgroup. The survivors in RACHS-1 subgroups 1 to 3 had lower mean lactate levels than the nonsurvivors in this group (P = 0.011), and this also held true for the survivors and nonsurvivors in RACHS-1 subgroups 4 to 6 (P = 0.026). Lactate levels differed significantly between survivors and nonsurvivors within the same RACHS-1 subgroup. This combination allows the targeting of appropriately intensive interventions and therapies toward the sickest patients.