The Modification of Diet in Renal Disease (MDRD) Study is an NIH sponsored multicenter clinical trial, the objective of which is to determine whether diet containing markedly reduced amounts of protein and phosphorus, with or without supplementation with a mixture of essential ketoacids and amino acids, will prove effective in reducing the rate of loss of renal function in patients with progressive renal insufficiency. Other objectives of the trial are to determine the nutritional safety of these diets and to determine the degree of patient acceptance of them. To date, in Phase I, a group of nine clinical centers and a data coordinating center prepared a protocol for Phase II, the feasibility trial, and Phase II, the feasibility trial itself, has been almost completed. Most recently the final protocol for Phase III has been completed. This protocol stipulates that after a three month period o baseline observation, patients may be entered into one of two branches of the trial depending upon initial glomerular filtration rate (GFR) measured as urinary clearance of 125I-iothalamate. Study A consists of those patients with GFR values between 60 and 25 ml/min, while Study B includes patients with GFR values between 25 and 10 ml/min. Study A patients are randomized into two arms, one receiving a diet wit relatively normal protein content (1.1 - 1.4 g/kg /day) and one receiving a diet with reduced protein content (0.58 g/kg/day). Study B patients will be randomized to receive the above reduced protein diet or to receive a diet even more markedly reduced in protein (0.28 g/kg/day), with an essential keto/amino acid supplement. Outcome will be determined in the two studies by comparison of the rates of loss of renal function (GFR) measured as above. Compliance and dietary acceptance will be monitored, by dietary records and by measurement of urinary nitrogen appearance rate. Nutritional status will be followed by measurement of serum proteins and by anthropometric measurements. The current application is for Phase III of the trial. We anticipate that we shall be able to enter 60 patients into the baseline period, from which at least 20 will be entered into each of Study A and Study B. Patients, will be identified within the patient population of the University of Iowa Hospitals by computer based screening of the Central Chemistry Laboratory creatinine determinations. In addition six regional nephrologists, 5 HMOs, and a clinic caring for low income patients have agreed to refer patients.