Proteinuria in diabetes is associated with progressive glomerular damage. We studied the effects of 3-wk dietary protein restriction on proteinuria and renal function in 10 insulin-dependent diabetic men with diabetic nephropathy. Patients were randomly assigned by a crossover design to 40-g low-protein diet (LPD) or usual-protein diet (UPD). Glomerular filtration rate and renal plasma flow were measured by inulin and p-aminohippurate clearance at the end of each period under conditions of sustained euglycemia. Total calorie intake, body weight, serum albumin and total protein concentrations, hematocrit, blood pressure, and glucose control were similar during the two diets. Achieved protein intake was 46 +/- 3 g/day during LPD and 81 +/- 4 g/day during UPD (P less than .001). Urinary urea appearance and plasma urea were significantly lower on LPD. Median total urinary protein was reduced from 3.9 g/day (range 0.5-12.3) on UPD to 2.4 (range 0.2-9.0) on LPD (P less than .006), and there was a significant fall in the median fractional clearance of albumin from 2.0 x 10(-4) (range 0.1-90.9) on UPD to 1.0 x 10(-4) (range 0.1-51.4) on LPD and IgG from 2.1 x 10(-5) (range 0.2-238) to 1.5 x 10(-5) (range 0.1-77) (P less than .006 and P less than .02, respectively). The reabsorption rate of beta 2-microglobulin was similar on the two diets and glomerular filtration rate, renal plasma flow, and filtration fraction remained unchanged. Thus, short-term dietary protein restriction reduces diabetic proteinuria independently of blood glucose or systemic blood pressure changes by improving glomerular permselectivity.