Serum concentrations of vitamins A and E and retinol-binding protein (RBP) were measured in 25 late adolescent and young adult patients with insulin-dependent diabetes mellitus. Their serum vitamin A levels were significantly lower than those of nondiabetic control subjects of comparable age. The serum concentrations of RBP were also significantly lower in the diabetic patients. The serum levels of vitamin A in the diabetic patients as well as in the control subjects showed a significant linear regression with serum concentrations of RBP. Unlike vitamin A, serum concentrations of vitamin E were not significantly different between the two groups of subjects. These findings suggest that the reduced serum vitamin A levels in the diabetic patients reflect reduced mobilization of vitamin A from the liver.