An early restriction of dietary protein intake may reduce the rate of renal function deterioration in human diabetics. This study investigates the effect of a restricted-protein diet on proteinuria in diabetic patients with persistent albuminuria after frequent dietary instruction. Twenty-six patients were divided into two groups: the restricted-protein diet (RPD) group (16 patients), who were on a diet containing 0.6-0.8 g/kg/day protein; and the normal protein diet (NPD) group (10 patients), who were on a diet containing 1.5 g/kg/day protein. Weekly dietary interview were conducted by a dietitian and a trained nurse. Of the 26 patients studied, five RPD patients and three NPD patients were dismissed. Of the 11 RPD patients who completed the study, seven patients (RPD-success) had a decreased calculated protein intake (mean value, 62 +/- 4 g/day) compared to the corresponding value (mean value, 73 +/- 5 g/day) before the study. The mean calculated protein intake value in the RPD-success group was higher than the protein content (42 +/- 1 g/day) prescribed for them; however, that of the NPD group, did not significantly differ before and at the end of the study. The RPD-success group's daily urinary protein excretion showed a significant reduction, with a mean value before (1378 +/- 414 mg/day) and after (880 +/- 413 mg/day) 3.7 +/- 0.4 months dietary protein restriction. In contrast, the daily protein loss in the NPD group was insignificant (1300 +/- 443 to 2976 +/- 862 mg/day) after 3.3 +/- 0.6 months dietary instruction.(ABSTRACT TRUNCATED AT 250 WORDS)