Obesity is associated with glomerular hyperfiltration and increased urinary protein excretion, as well as structural and functional changes that lead to kidney disease and failure. Dietary protein mimics obesity's effects on the glomerular filtration rate (GFR) and proteinuria and, in certain circumstances, may have the potential to adversely affect kidney function. Here we tested the hypothesis that dietary protein independently explains elevations in the GFR and proteinuria found in obese persons with a normal serum creatinine. Seventeen patients were randomized in a double-blind, crossover fashion for 1-week periods to high (140 g/day) and low (50 g/day) protein diets with a 1-week washout interval separating these periods. High protein consumption was associated with a very modest but significant increase in the GFR of 5 ± 6 ml/min. Hence, while dietary protein does modulate kidney parameters, it is unlikely to fully account for the elevations in GFR and proteinuria found in obesity.