1. Thirty patients with acute renal failure who were unable to eat adequately were evaluated while they received parenteral nutrition with glucose alone (n = 7), glucose and 21 g/day essential amino acids (EAA, n = 11) or glucose, 21 g/day essential and 21 g/day nonessential amino acids (ENAA, n = 12). Energy intake did not differ with the three treatments. Patients were studied in a prospective double blind fashion. 2. Thirteen patients recovered renal function and 11 survived to leave the hospital. Those in whom renal failure was attributed to hypotension and/or sepsis had a poorer recovery of renal function (17%) and survival (17%). Recovery of renal function and survival was greater in patients on the medical service as compared to the surgical service and in those who received more energy. Recovery of renal function was worse in those treated with dialysis. There were no differences in recovery of renal function of survival among the three treatment groups. 3. Many patients were markedly catabolic as indicated by nitrogen balances, urea in nitrogen appearance rates (UNA), serum protein concentrations, and plasma amino acid levels. There was no correlation between the degree of catabolism and recovery of renal function or survival. Mean UNA in individual patients also correlated with body weight. Among the three groups, however, UNA was significantly less with the group receiving EAA as compared to ENAA. 4. Serum protein concentrations were lower than normal in all treatment groups. Serum albumin fell significantly during the treatment in the more catabolic patients. Plasma amino acid levels tended to fall in all three groups and concentrations at the end of the treatment were frequently lower than normal. 5. These data suggest that acute renal failure patients who are unable to eat adequately are often hypercatabolic and have a high mortality, particularly if hypotension or sepsis is the cause of renal failure. The improved survival in those with higher energy intakes, the high rate of net protein breakdown, the low serum protein levels and the reduced plasma concentrations of both essential and nonessential amino acids suggest that greater quantities of energy and both essential and nonessential amino acids may be beneficial to such patients.