A case-control study was performed to establish possible risk factors for acute renal failure (ARF) and mortality in patients undergoing cardiac surgery. A consecutive series of 704 patients were included in the study. A randomized sample of 255 patients was taken to analyze risk factors for ARF and mortality. Incidence of acute renal failure was 3.8% (27/704). Low cardiac output (LCO) was observed in all patients who developed ARF and in 72/255 without ARF (p < 0.0005). When LCO was isolated, no difference was found between groups. Association to prolonged perfusion time and sepsis increased the probability of developing ARF: 5/27 versus 0/255, p < 0.001; and 9/27 versus 0/255, p < 0.001, respectively. Overall mortality was 7.2% (51/704). Significant difference was found between ARF (55.6%, 15/27) and non-ARF patients (5.4%, 36/704), p < 0.005. Neither age nor sex was associated to outcome. Nonsurvivor patients were more oliguric (11/15 vs. 0/12, p < 0.005), required dialysis more frequently (7/15 vs. 0/12, p < 0.005), and were complicated with sepsis more often (9/15 vs. 0/12, p < 0.005), compared to survivors. We concluded that ARF was an uncommon complication in this group of patients, but mortality rate was dramatically high. This study identified LCO associated to prolonged perfusion time and sepsis as risk factors for ARF. Severity of ARF (oliguric forms and dialysis requirement) and postoperative events (sepsis) were associated with mortality.