The progression of renal failure on 2 different protein and phosphate diets was evaluated in 7 patients with chronic renal failure. We decided on three study periods for each patient: period A (low-protein diet: 0.6 g/kg of protein, 600 mg of phosphate) averaged 12 months; period B (normal protein diet: 0.9 g/kg of protein, 800 mg of phosphate) lasted 9 months; period C (low-protein diet as for period A) averaged 9 months. The slopes of 1/Scr were 0.003 (period A), -0.016 (period B) and 0.009 dl/mg/month (period C). The GFR (125I-iothalamate clearance) changed from 42.6 to 45.6 ml/min/1.73 m2 in period A, from 51.9 to 40.7 in period B and from 40.5 to 44.6 in period C. The results show changes in renal function when dietary protein and phosphate intakes exceeding 0.6 g/kg and 600 mg/day, respectively, are administered to patients with GFRs ranging from 24 to 66 ml/min.