Arandomized, controlled study of 12 patients with mild chronic renal failure was designed to assess the metabolic effects of a low-protein diet supplemented (n = 6) or not (n = 6) with ketoanalogs of amino acids. The protein intake was prescribed so that both groups were isonitrogenous. The dietary survey each month included a 3-d food record and a 24-h urine collection for urea measurement. After a 4- to 6-wk equilibrium period (standard occidental diet, 1.11 g of protein and 32 kcal/kg per d), patients reduced their protein intake to reach 0.71 g of protein/kg per d during the third month. Energy intake was kept constant (31 kcal/kg per d) during the 3-mo period. Compliance to the diet was achieved after 2 mo of training. Leucine turnover measurement was performed before and at the end of the 3-mo low-protein period. There was no clinical change, whereas total body flux decreased by 8% (P < 0.05) and leucine oxidation by 18% (P < 0.05). No difference could be attributed to the ketoanalogs themselves. Thus, under sufficient energy intake, a low-protein diet is nutritionally and metabolically safe during chronic renal failure. The nitrogen-sparing effect of a low-protein diet is still present during mild chronic renal insufficiency.