The suggested harmful effect of dietary protein on renal function in diabetic nephropathy was tested in three groups of insulin-dependent diabetic patients: 1) 10 patients without signs of nephropathy in spite of at least 30 years of diabetes; 2) 11 patients with nephropathy and reduced but stable glomerular filtration rate (GFR) (decline less than 4 ml/min per year [mean 1.8] during the last 2 years); 3) 10 patients with progressive nephropathy with GFR declining by an average of 11 ml/min per year. Dietary protein intake was estimated from a dietary history interview, as well as from urinary excretion of nitrogen (mean = 4.7 samples). Both methods showed a wide range of protein intake in all three groups of patients (0.6-2.3 g/kg body weight [BW]). The mean values did not differ between the groups, 1.30, 1.34, and 1.24 g/kg BW by interview, and 1.20, 1.10, and 1.13 g based on urinary nitrogen levels. There was no correlation between rate of decline of GFR and protein intake, even in those patients with no or minimal decline. These results do not support the hypothesis that dietary protein is a factor of importance in the development or progression of human diabetic nephropathy.