Acute renal failure often has a complex origin, especially in critically ill patient. Moreover, the tools for investigation of renal failure are generally unspecific, so that the work-up must be based on several elements. It is particularly important to differentiate failure of the kidney itself from prerenal or postrenal failure, since the therapeutic implications are very different. We propose to base the investigation of acute renal failure on some elements to be evaluated in a rapid succession. The complete evaluation should also take into account the consequences of renal failure, which also have therapeutic implications.