To determine the effectiveness of dietary protein restriction on proteinuria in patients with non-insulin dependent diabetes (NIDDM), 14 diabetic patients with overt nephropathy were placed on either a low protein diet (N = 7) or conventional protein diet (N = 7) for one month. After the study period, daily urinary protein excretion rates decreased significantly, from 3.2 +/- 0.4 to 1.9 +/- 0.4 g/day, and serum albumin levels increased from 3.3 +/- 0.2 to 3.7 +/- 0.5 g/dl only in the low protein diet group, without any significant changes in either serum creatinine levels or creatinine clearance. These findings suggest that dietary protein restriction has a beneficial role in the treatment of NIDDM patients with overt nephropathy.