Free radicals are considered to be important factors involved in many physiopathological processes. Several methods have been proposed for studying the mechanisms of antioxidant protection against free radical-induced injury, including the measurement of the total antioxidant capacity (TAC) in body fluids, based on enhanced chemiluminescence. This technique is calibrated against Trolox and assay results are expressed as mumol/L of Trolox equivalents. Since many of the complications induced by diabetes appear to be mediated by oxygen free radical generation, we have investigated serum antioxidant capacity in a group of healthy subjects and in insulin-dependent diabetic (IDDM) subjects. A statistically significant difference was noticed in TAC values between the IDDM group and the young control group. Even if the biological meaning of this significant reduction in TAC remains to be explained, an overproduction of precursors of reactive oxygen free radicals and/or a decreased scavenger systems efficiency can be associated with the increased risk of atherosclerotic cardiovascular disease in diabetic patients.