There have been a large number of clinical trials and two meta-analyses attempting to determine if dietary protein restriction retards the rate of renal disease progression. Indeed, in a systematic search of prospective, controlled trials examining the effects of protein restriction over at least 6 months, we located 23 published between 1980 and 1996. Only 12 of these trials were randomized and controlled. The results of the 23 studies were heterogeneous. Two meta-analyses combined the results of randomized, controlled trials. Although both meta-analyses concluded that dietary protein restriction significantly reduced the number of patients who died or required treatment for end-stage renal disease, the well-known effects of protein restriction on the signs and symptoms of uremia leave open the question of whether protein restriction had a substantial effect on renal disease progression per se. Adding to the uncertainty is the inconclusive result of the largest, best-designed clinical trial, the Modification of Diet in Renal Disease Study. In any case, the effect of protein restriction on the rate of decline in renal function is arguably modest, making it difficult to demonstrate statistical significance, even in large, well-designed clinical trials. All of these trials suggest that we need therapies that are more effective than dietary protein restriction to halt the progression of renal disease.