Background: Chronic kidney disease (CKD) is a frequent and serious complication of cardiac surgery. This study was designed to establish a scoring system, calculated in the immediate postoperative period, to assess the risk of CKD at 1 yr in patients undergoing cardiac surgery with cardiopulmonary bypass. Methods: We conducted a cohort study including patients with preoperative estimated glomerular filtration rate above 60 ml min-1 (1.73 m)-2 who underwent cardiac surgery with cardiopulmonary bypass. We identified risk factors for de novo CKD at 1 yr using logistic regression. We derived a risk score for CKD, and externally validated this score in a second cohort. Results: The incidence of CKD was 18% and 23% in the derivation and validation cohorts, respectively. We developed a scoring system that included (i) the occurrence of postoperative acute kidney injury according to the Kidney Disease: Improving Global Outcomes criteria, (ii) age older than 65 yr, (iii) preoperative glomerular filtration rate <80 ml min-1 (1.73 m)-2, (iv) aortic cross-clamping time longer than 50 min, and (v) the type of surgery (aortic or cardiac transplantation). This score predicted CKD with good accuracy (area under the receiver operating characteristic curve: 0.81; 95% confidence interval: 0.77-0.86 in the derivation cohort), and with fair accuracy in the validation cohort (area under the receiver operating characteristic curve: 0.78; 95% confidence interval: 0.72-0.83). Conclusions: We provide an easy-to-calculate scoring system to identify patients at high risk of developing CKD after cardiac surgery with cardiopulmonary bypass. This system might help clinicians to target more accurately patients requiring monitoring of renal function after cardiac surgery, and to design appropriate interventional trials aimed at preventing CKD or mitigating its consequences.