The aim of the study was to investigate the influence of intrarenal RAS on the decrease of renal function in patients undergoing cardiac surgery with cardiopulmonary bypass. This observational study investigated the activation of intrarenal RAS in 24 patients with AKI after cardiac surgery with cardiopulmonary bypass. The activation of intrarenal RAS was determined by urinary angiotensinogen (uAGT), which was measured at 12 hours before surgery, 0 and12 hours after surgery. The results were compared with those of 21 patients without AKI after cardiac surgery with cardiopulmonary bypass. Clinical and laboratory data were collected. Compared with baseline, all patients with cardiac surgery had activation of intrarenal RAS at 0 and 12 hours after surgery. The activation of intrarenal RAS was found significantly higher at both 0 and 12 hours after surgery in AKI group versus non AKI group (6.18 ± 1.93 ng/mL vs 3.49 ± 1.71 ng/mL, 16.38 ± 7.50 ng/mL vs 6.04 ± 2.59 ng/mL, respectively). There was a positive correlation between the activation of RAS at 0 hour after surgery and the decrease of renal function at 48 hours after surgery (r = 0.654, P = .001). These findings suggest that uAGT might be a suitable biomarker for prediction of the occurrence and severity of AKI after cardiac surgery. Inhibition of intrarenal RAS activation might be one the path of future treatment for this type of disease.