To examine the influence of oxidative stress on oxidative protein damage, we studied 51 young Type 1 diabetic patients clinically free of complications and 48 healthy normolipidaemic age-matched controls. We determined: (1) plasma carbonyl (PCO), plasma total thiol (T-SH), and nitrotyrosine (NT) levels as markers of oxidative protein damage; (2) plasma lipid hydroperoxide (LHP), and nitric oxide (NO) levels as markers of oxidative stress; (3) plasma total antioxidant capacity (TAO), ceruloplasmin (Cp), transferrin (TRF), unsaturated iron binding capacity (UIBC), erythrocyte glutathione (GSH), and erythrocyte superoxide dismutase (SOD) as markers of free radical scavengers. There were no significant differences in the levels of these markers between prepubertal diabetic patients and the controls. The levels of both of PCO and LHP were increased in adolescent and young adult Type 1 diabetic patients with respect to their controls. In the adolescent group, patient versus control values for PCO were 1.04 +/- 0.067 versus 0.67 +/- 0.0274 nmol/mg and for LHP they were 2.10 +/- 1.09 versus 1.00 +/- 0.4 nmol/mg. In the young adult group, patient versus control values for PCO were 0.99 +/- 0.054 versus 0.66+/-0.02 nmol/mg and for LHP they were 1.96+/-0.78 versus 1.15+/-0.4 nmol/mg. TAO levels were significantly decreased in adolescent diabetic patients compared to their controls (0.92 +/- 0.27 vs. 1.86 +/- 0.37) and in young adult diabetic patients compared to their controls (0.80 +/- 0.27 vs. 2.11 +/- 0.54 nmol/mg). T-SH was not different between diabetic patients and the controls. Serum NT, NO, and erythrocyte SOD levels were not different either between three groups of diabetic patients or between the patients and their controls. We attribute this lack of difference to limited disease duration. Changes in markers of oxidative stress other than NT, NO, and SOD observed in adolescent and young adult early stage Type 1 diabetic patients contribute to the imbalance in the redox status of the plasma. We attribute this imbalance to metal-catalyzed protein oxidation in both groups of Type 1 diabetic patients clinically free from complications. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.