Acute kidney injury (AKI) is a frequent complication of cardiopulmonary bypass. The paucity of early, predictive, non-invasive biomarkers has impaired our ability to institute potentially effective therapy as soon as possible. The most promising new biomarker is neutrophil gelatinase-associated lipocalin (NGAL). The aim of the study was to assess the urine and serum level of NGAL in children undergoing cardiopulmonary bypass (CPB) due to severe congenital heart disease. Material and methods: We prospectively studied 48 children (17 girls and 31 boys) after CPB, which were divided in two group: with and without acute kidney injury defined as a 50% increase in serum creatinine from baseline. Serum and urine samples were collected before and after 2, 4, 12, 24, 48 hours of the CPB beginning. Results: Twenty-four (54%) children developed AKI. Serum NGAL rose from 26 ng/ml at baseline to 60 ng/ml after 2 and 65 ng/ml after 4 hours post-procedure outstripping creatinine rise (12 hours). Serum concentration was not different in both group (AKI and non-AKI). Urine concentration of NGAL rose in differently in both group (AKI, non-AKI) in 2 (median 62 vs 27 ng/ml; p=0.01) and 4 hours (median 28 vs 13 ng/ml; p=0.056). Conclusion: Urine and serum concentration of NGAL raises in children undergoing cardiopulmonary bypass. NGAL can be useful as a early biomarker of acute kidney injury.