In a prospective randomised study of 228 patients with various renal diseases, early moderate dietary protein restriction retarded the development of end-stage renal failure. 149 patients were followed up for at least 18 months; the protein-restricted patients showed falls in serum urea and phosphate concentrations and in the 24 h excretion of urea, phosphate, and protein. Regression analysis of the reciprocals of serum creatinine against time showed that the average rate of decrease in reciprocal creatinine was three to five times lower in the protein-restricted groups than in the control groups. These results confirm that moderate dietary protein restriction is an acceptable and effective way of delaying functional renal deterioration. The finding has implications for the management of chronic renal insufficiency.