Objective[|]In this present study, the association of hyperlactatemia frequency with probable risk factors, postoperative morbidity and mortality were investigated in patients undergoing paediatric open heart surgery.[¤]Materials and Methods[|]The present study included 45 consecutive paediatric patients who had undergone open heart surgery with hypothermic cardiopulmonary bypass in the cardiovascular surgery clinic between January 2008 and July 2008. Four blood samples for lactate analysis were collected from each of the patients preoperatively, intraoperatively and at 1 and 12 hours postoperatively. The patients were divided into two groups according to blood lactate levels as the high lactate group (mean lactate level≥ 3 mmol/L) and the normal lactate group (mean lactate level <3 mmol/L). Hyperlactatemia frequency, associated risk factors, and the relationship with morbidity and mortality were statistically analysed.[¤]Results[|]Of 45 cases, 33 (73.3%) were included in the normal lactate (NL) group, and 12 (26.7%) were included in the high lactate (HL) group. A borderline association was found between lactate levels and mortality in the HL group (p=0.052). Body surface area, age, low cardiac output syndrome, intraoperative and postoperative inotropic support requirement, duration of mechanical ventilation were determined as risk factors associated with mortality (p<0.05), and low cardiac output syndrome, urine output and metabolic acidosis were determined as risk factors associated with hyperlactatemia (p<0.05).[¤]Conclusion[|]In patients followed up in the intensive care unit, lactate concentration is a good indicator for disease severity. Blood lactate levels seems to be a parameter that can be used in routine follow-up.[¤]