Diabetes mellitus is associated with endothelial dysfunction and oxidative stress (OS). We investigated whether these abnormalities are interrelated in children and adolescents with type 1 diabetes mellitus (T1DM) and if early OS markers predictive of vascular dysfunction can be identified. Thirty-five T1DM patients were matched for sex, age, height, and weight with nondiabetic subjects as healthy controls (CO). Flow-mediated dilatation (FMD), carotid intima media thickness (IMT), and OS status in fasting blood were measured. Diabetic children had impaired FMD (6.68+/-1.98 versus 7.92+/-1.60% in CO, p=0.004), which was more pronounced in boys. The degree of FMD impairment was not related to the lower plasma levels of antioxidants or to the higher glucose, glycation, lipids, and peroxidation products. Erythrocyte superoxide dismutase activity, copper/zinc superoxide dismutase (Cu/Zn SOD), was higher in diabetic subjects (1008+/-224 versus 845+/-195 U/g Hb in CO, p=0.003) and was positively associated with FMD. After correcting for diabetes and gender, the subgroup of children with high Cu/Zn SOD (>955 U/g Hb) had a significantly better FMD (p=0.035). These results suggest that higher circulating Cu/Zn SOD could protect T1DM children and adolescents against endothelial dysfunction. Low Cu/Zn SOD is a potential early marker of susceptibility to diabetic vascular disease.