The possible influence of C-peptide on renal function and metabolic control in patients with type 1 diabetes was examined in a double blind, randomized study. Nine patients received insulin and equimolar amounts of biosynthetic human C-peptide for 1 month (group 1), and nine were given insulin only (group 2). C-Peptide levels in plasma ranged from 0.3-2.6 nmol/L in group 1 during the study, whereas group 2 had undetectable levels. The urinary excretion of albumin in group 1 was 21 +/- 6 micrograms/min before the study and decreased by 40% and 55% after 2 and 4 weeks, respectively (P < 0.05). No change was seen in group 2. The glomerular filtration rate fell by 6% after 2 and 4 weeks (P < 0.05) in group 1, whereas no change was observed in group 2. Fluorescein leakage across the blood-retinal barrier decreased by 30% in group 1 (P < 0.05) and was unaltered in group 2. Hemoglobin-A1c and fructosamine values decreased by 9-16% in group 1 (P < 0.05), but not in group 2. The findings suggest that administration of C-peptide plus insulin, compared to insulin alone, to type 1 diabetic patients may reduce glomerular permeability and improve metabolic control.