There are now powerful compensatory therapies to counteract kidney deficiency and the prognosis of patients with acute renal failure is mainly related to the severity of the initial disease. Renal failure is accompanied by an increase in both severity and duration of the catabolic phase leading to stronger catabolic consequences. The specificity of the metabolic and nutritional disorders in the most severely ill patients is the consequence of three additive phenomena: (1) the metabolic response to stress and to organ dysfunction, (2) the lack of normal kidney function and (3) the interference with the renal treatment (hemodialysis, hemofiltration or both, continuous or intermittent, lactate or bicarbonate buffer, etc.). As in many other diseases of similar severity, adequate nutritional support in acutely ill patients with ARF is of great interest in clinical practice, although the real improvement as a result of this support is still difficult to assess in terms of morbidity or mortality.