Background: Cardiopulmonary bypass (CPB) affects almost every body system by hypoperfusion either subclinically or clinically and produces a systemic inflammatory response owing to contact of blood with mechanical surfaces. There has been no documentation regarding pyruvate in predicting postoperative mortality and morbidity. Aim: This study was carried out to evaluate lactate levels, pyruvate levels, and lactate-pyruvate ratio in pediatric patients undergoing CPB (for correction of congenital cardiac anomaly) and their correlation to perioperative outcomes. Method: 50 consecutive patients (less than 14 years of age) of various congenital heart diseases undergoing CPB were studied. Patients were classified into three categories according to their surgical complexity. Arterial blood samples were collected at different stages of CPB to estimate blood lactate and pyruvate levels by using documented spectrophotometric method. Results: The observed mean baseline lactate levels were 2.24 + 0.79 mmol/l (normal range of 0.9-1.7 mmol/l). The mean circulating lactate levels, at 15 and 45 min after institution of CPB, increased to 4.49 + 1.2 and 5.24 + 1.02 mmol/l, respectively. A progressive decline in the mean lactate levels was noted during rewarming (at 35°C) and immediately off-bypass which continued steadily even 24 and 48 h postoperatively. The mean baseline lactate-pyruvate ratio was 24.73, which increased at 15 and 45 min after institution of CPB, rewarming (at 35°C), and immediately off-bypass. As far as the duration of CPB is concerned, we found that lactate levels were elevated significantly (P < 0.05) and the lactate-pyruvate ratio was significantly high (P < 0.001) during rewarming, off-bypass, and 24- and 48-h post-CPB in patients requiring CPB for more than 1 h. The average duration of postoperative mechanical ventilation, inotropic support, and lactate-pyruvate ratio were significantly higher (P < 0.001) in category III of patients in comparisons with categories I and II. Conclusion: In patients undergoing CPB for congenital heart operation, elevations in lactate-pyruvate ratio can predict the postoperative outcome significantly better in comparison with lactate levels.