Unrestricted intake of protein-rich foods is accompanied by sustained increases in glomerular capillary pressures and flows. Intrarenal hypertension and hyperperfusion associated with protein intake may eventually cause glomerular sclerosis and account for decreased renal function seen with aging. Further elevation of glomerular capillary pressures and flows contributes to progressive glomerular destruction and eventual loss of renal function when nephron number has been reduced by renal disease. Progressive loss of renal function may be retarded by restriction of protein intake. Protein restriction appears to preserve renal function by limiting intrarenal capillary hypertension and hyperperfusion.