Acute renal failure (ARF) develops in 1-30% of patients who undergo cardiac surgery and is associated with a high mortality rate (15-30%). Several risk factors (pre- and intra-operative) for ARF have been identified. Pre-operative factors are strictly related to cardiovascular disease, advanced age and baseline renal dysfunction, while intra-operative factors are linked with the type of cardiac surgery, the duration of cardiopulmonary bypass and aortic cross-clamping. These factors provide an opportunity to quantify the risk of ARF based on pre-operative data, and for this purpose a clinical score to predict post-operative ARF has recently been developed. Moreover, this score could allow the identification of those patients who may take advantage of preventive strategies. Mortality in patients who develop severe ARF requiring dialysis is particularly high (50-80%). Therefore, an early diagnosis of ARF and a timely and aggressive renal replacement therapy could improve the outcome.