An investigation was made into whether vitamin C supplementation is effective on urinary albumin excretion rate in patients with insulin dependent diabetes mellitus who have microalbuminuria. Vitamin C 1000 mg daily was instituted for the normotensive IDDM patients with microalbuminuria (n=15). Albumin excretion rates and metabolic parameters of the patients were examined before and at the end of the 3rd month of vitamin C supplementation. Urinary albumin excretion was meausured in 24 hr-urine samples by a radioimmunoassay method and was estimated as µg/min. The mean duration of diabetes of the patients was 13.3±1.8 yr. (4-30 yr). Blood urea nitrogen, creatinine and creatinine clearance were normal and after vitamin C supplementation, they were not changed. In the 3rd month of vitamin C supplementation, glycosylated hemoglobin level was lower than that at the beginning but the difference was not statistically significant (8.9±0.7% vs 9.3±0.3% respectively, P=NS). Urinary albumin excretion rate was reduced from 84.8±15.7 µg/min to 60.3±11.5 µg/min throughout vitamin C supplementation and this difference was statistically significant (P