Measurement of lactate concentrations during cardiac surgery with cardiopulmonary bypass (CPB) is a reliable monitoring tool for the assessment of the adequacy of perfusion, and a predictor of poor outcome. However, increased lactate production, which is multifactorial (anaerobic metabolism, hyperglycemia), increased lactate load by packed red blood cell (PRBC) transfusions, and decreased lactate clearance may all result in hyperlactatemia. The aim of this study was to estimate the clearance of lactate in infants undergoing surgery with CPB, using the lactate load from the PRBCs transfusions received during CPB. Retrospective cohort of infants <1 year of age with repeated lactate measurements during CPB, and a known lactate concentration in the PRBCs used during CPB were evaluated. All patients received PRBCs in the prime and during CPB to maintain hematocrit >35% and venous saturation >70%. Lactate kinetics were estimated across several time intervals between two lactate measurements, using a single compartment model. The lactate load was calculated as the product: PRBC‐lactate concentration * volume. The rate of endogenous lactate production was assumed to be unchanged (maintenance of high oxygen deliveries and normoglycemia throughout CPB). Overall, 87 calculations were performed in 27 patients, then averaged per patient. The mean lactate half‐life was 12.36 min [10.67–14.06], the mean clearance was 0.09 L/min [0.06–0.11], the indexed lactate clearance was 0.36 L/min/m2 [0.28–0.44]. Lactate clearance increased significantly with age. The half‐life of lactate in infants is comparable with that reported in adults with CPB, and lactate clearance is higher. Knowing the high lactate content of PRBCs, lactate clearance rather than absolute concentration is potentially a better indicator of the adequacy of perfusion during CPB in infants.