Background: Acute kidney injury (AKI) is a common complication following cardiac surgery. Studying the correlation between hematological indices and the occurrence of postoperative AKI following cardiac surgery seems to be important to predict the incidence of AKI and subsequently improving the outcome. Aim of The Work: To evaluate the potential utility of the hematological indices for predicting postoperative AKI in the background of cardiac surgery. Patients and methods: This observational, cohort, prospective study was conducted at Cardiothoracic Department and ICU, Zagazig University Hospitals, Egypt , in the period from Jan. to Oct , 2023 , on 126 patients underwent cardiothoracic surgery. The patients were classified into group 1: (63 patients with no AKI after cardiac surgery) and group 2: (63 patients with AKI after cardiac surgery). The hematological ratios: neutrophil to lymphocyte ratio (NLR), systemic immune-inflammation (SII) index were calculated. Results: In multivariate regression analysis, platelets, lymphocytes, NLR, SII, creatinine, and BUN were significant predictors for AKI in patients who underwent valvular replacement. In multivariate regression analysis, BUN was a significant predictor for AKI in patients who underwent CABG. There is a correlation between increased NLR in the early postoperative stage and CSA-AKI. Conclusion: NLR, as a surrogate of an inflammatory response, could be valuable as a marker for the timely prevention of AKI.