The rate of progression of renal failure has been evaluated in two homogenous groups of chronic renal patients with early insufficiency. In both groups the diet supplied the same amount of calories (approximately 35 Kcal/kg/day) and the protein intake was equally restricted (approximately, 0.6 g/kg/day); however, in Group 1 the phosphorus intake was lower (6.5 mg/kg/day) than in Group 2 (12 mg/kg/day). In both groups the rate of decline of creatinine clearance decreased when patients changed from a free mixed diet to the specially controlled diets, but in Group 1 (lower phosphorus intake) this change was much more marked than in Group 2. Elevated mean levels of serum phosphate and of urinary output of phosphate per unit of creatinine clearance, and elevated mean levels of serum iPTH were found in the patients of Group 2, whereas Group 1 patients had normal mean levels of serum phosphate and of iPTH, and the phosphaturia per unit of creatinine clearance was almost normal. The role of such abnormalities in urinary and serum phosphate, and of secondary hyperparathyroidism, on the progression of renal failure is discussed.