Two findings prompted investigators to examine the effects of dietary manipulation on progression of chronic renal failure: dietary protein restriction is an effective method of ameliorating uremic symptoms and the course of renal insufficiency in an individual patient is predictable. Results from studies of patients and animals with chronic renal failure suggested that a low-protein, phosphorus-restricted diet could slow the rate of loss of renal function. In evaluating these studies, three questions should be considered. First, is the diet nutritionally adequate? Second, has dietary compliance been monitored and achieved? Third, is there evidence that restricting the diet will change the rate of loss of renal function? The scientific basis for each of these questions is addressed in this review.