Findings of inadequate tissue perfusion might be used to predict the risk of mortality. In this study, we evaluated the effects of lactate and lactate clearance on mortality of patients who had undergone extracorporeal membrane oxygenation (ECMO). Patients younger than 18 years old and who needed venoarterial ECMO support after surgery for congenital heart defects, from July 2010 to January 2019, were retrospectively analyzed. Patients successfully weaned from ECMO constituted Group 1, and patients who could not be weaned from ECMO were in Group 2. Postoperative clinics and follow-ups of the groups including mortality and discharge rates were evaluated. There were 1,844 congenital heart surgeries during the study period, and 55 patients that required ECMO support were included in the study. There was no statistically significant difference between the groups regarding demographics and operative variables. The sixth-, 12th-, and 24th-hour lactate levels in Group 1 were statistically significantly lower than those in Group 2 (P=0.046, P=0.024, and P<0.001, respectively). There were statistically significant differences regarding lactate clearance between the groups at the 24th hour (P=0.009). The cutoff point for lactate level was found as ≥ 2.9, with 74.07% sensitivity and 78.57% specificity (P<0.001). The cutoff point for lactate clearance was determined as 69.44%, with 59.26% sensitivity and 78.57% specificity (P=0.003). Prognostic predictive factors are important to initiate advanced treatment modalities in patients with ECMO support. In this condition, lactate and lactate clearance might be used as a predictive marker.