A total of 775 consecutive patients who survived the first 24 hours after cardiac operation were prospectively studied to assess the prevalence, mortality rate, and main risk factors for development of new acute renal failure. Normal renal function before operation (serum creatinine level less than 1.5 mg/dl) was registered in 734 (94.7%) patients. Of these, 111 (15.1%) showed a postoperative renal complication including 84 (11.4%) classified as renal dysfunction (serum creatinine level between 1.5 and 2.5 mg/dl) and 27 (3.7%) as acute renal failure (serum creatinine level higher than 2.5 mg/dl). The mortality rate was 0.8% in normal patients, 9.5% in patients with renal dysfunction, and 44.4% when acute renal failure developed (p < 0.0001). Indeed, the renal impairment proved to be an independent predictor of mortality (p < 0.001), along with the infective (p < 0.001), gastrointestinal (p < 0.001), and cardiovascular (p < 0.05) complications. Multivariate analysis identified the following variables as independent risk factors for postoperative renal impairment: use of intraaortic balloon pump (p < 0.0001), need for deep hypothermic circulatory arrest (p < 0.005), low-output syndrome (p < 0.005), advanced age (p < 0.005), need for emergency operation (p < 0.025), and low urinary output during cardiopulmonary bypass (p < 0.05). The 41 patients (5.3%) with preoperative renal failure showed a significantly higher morbidity and mortality rate than those without renal complications before operation. We conclude that in patients undergoing cardiac operation without preexisting renal dysfunction the likelihood of severe renal complications is reasonably low, but the associated mortality remains high. A prominent role in the development of postoperative acute renal failure must be recognized for preoperative, intraoperative, and postoperative hemodynamic factors, whereas cardiopulmonary bypass seems to be of lesser importance in this respect.