Dietary protein restriction has been reported to delay the need for renal replacement therapy in clinical trials and meta-analyses. However, less clear is what effect dietary protein has on the rate of decline in renal function. We pooled the results of 13 randomized controlled trials (n = 1,919 patients) and found that dietary protein restriction reduced the rate of decline in estimated glomerular filtration rate by only 0.53 mL/min/yr (95% confidence interval [CI], 0.08 to 0.98 mL/min/yr). We also used weighted regression analysis to determine the reasons for the differences in the results of these 13 randomized trials along with 11 other nonrandomized controlled trials (n = 2,248 patients). The effect of dietary protein restriction (glomerular filtration rate decline in treatment minus control) was substantially less in randomized versus nonrandomized trials (regression coefficient, -5.2 mL/min/yr; 95% CI, -7.8 to -2.5 mL/min/yr; P < 0.05) and relatively greater among diabetic versus nondiabetic patients (5.4 mL/min/yr; 95% CI, 0.3 to 10.5 mL/min/yr; P < 0.05), while there was a trend toward a greater effect with each additional year of follow-up (2.1 mL/min/yr; 95% CI, -0.05 to 4.2 mL/min/yr; P = NS). However, the number of diabetic patients studied was small and the duration of follow-up was short in most trials. No other patient or study characteristics altered the effect of dietary protein restriction on the rate of decline in renal function. Thus, although dietary protein restriction retards the rate of renal function decline, the relatively weak magnitude of this effect suggests that better therapies are needed to slow the rate of renal disease progression.