Acute kidney injury (AKI) signifiesfrequentcomplicationaftercardiacsurgeryusingcardiopulmonarybypass(CPB).AKIraises the risk for prospective chronic kidney disease and renal failure, with its related mortality and morbidity. Traditional diagnostic approaches to AKI diagnosis such as changes in serum creatinine and blood urea was delayed by 2 to 3 days after CPB. Early detection of acute kidney injury (AKI) after cardiac surgery may improve patient's outcome, potentially reducing mortality, hospital length of stay, and costs. In the hope to enhance earlier more reliable characterization of AKI, we tested the utility of urine lipocalin biomarker in addition to standard creatinine and blood urea for early determination of AKI after cardiac surgery using CPB. Thirty patients were enrolled in the current study. 9 patients (30%) developed AKI and 21 (70%) did not (non-AKI group). Groups were comparable regarding demographics and surgical features. Serum creatinine and blood urea levels showed no statistical differences between the two groups at 2 hours after surgery and later on the firstpostoperativeday. A significantelevationofcreatinineandbloodureaonlyobservedinthegroupwithAKIonthesecondpost-operativeday.Urinary lipocalin showed a highly significantdifferencebetweenthetwogroups2hoursafterCPB(P=<0.001)andlateronthe firstpostoperativeday(P<0.001).Elevatedlipocalinlevelssignificantlycorrelatedwithlongercardiopulmonarybypass,longer hospital stay, and death. From this study, it could be concluded that urinary lipocalin was a powerful predictor of acute kidney injury following CPB surgery.