To evaluate oxidative stress in type I diabetes mellitus, two antioxidant enzymes in erythrocytes, copper-zinc superoxide dismutase (SOD EC 1.15.1.1.) and seleno-dependent glutathione peroxidase (GSH-Px; EC 1.11.19), and two indexes of peroxidation in plasma, thiobarbituric acid reactive substances (TBARS) and organic hydroperoxides (OHP), were measured in 118 patients with insulin-dependent diabetes mellitus (IDDM), classified in accordance with the presence or absence of vascular complications and the degree of metabolic control established by the HbA1c level. Ninety healthy subjects made up the control group. According to our results, plasmatic TBARS and OHP concentrations are significantly higher in diabetics than in controls, and these differences are accentuated in diabetic people with vascular disorders. The GSH-Px activity was significantly reduced in diabetic patients with poor and medium metabolic control in relation to the control group, regardless of the existence or absence of vascular disorders. No differences in SOD activity between diabetic and control groups were found. A significant positive correlation between TBARS and HPO (r=0.683, p<0.001) was found in both the control and diabetic groups. Among the lipid parameters studied, there were only significantly positive correlations between TBARS and total cholesterol; TBARS and triglycerides; OHP and total cholesterol and OHP and triglycerides. Positive correlations between TBARS and HbA1c and between OHP and and HbA1c, and negative correlations between GSH-Px and HbA1c and between SOD and HbA1c were also found. The multiple regression analysis shows that TBARS and HPO correlate negatively with GSH-Px. There was no significant correlation with SOD.