In this study, we evaluated protein oxidation in 84 patients with Type 2 diabetes with no complications and in 61 healthy volunteers who formed the control group, whose ages matched those of the patients. We determined plasma carbonyl and plasma thiol levels as markers of oxidative protein damage and erythrocyte glutathione, plasma ceruloplasmin and transferrin as markers of free radical scavengers. The concentrations (mean ± SD) of both of plasma carbonyl (1.24 ± 0.46 vs. 0.72 ± 0.17 nmole/mg protein; p < 0.0001) and lipid hydroperoxides (1.8 ± 0.63 vs. 1.3 ± 0.21 µtmole/l; p<0.0001) were increased, and the concentration of plasma transferrin (3.85 ± 0.65 vs. 4.59 ± 0.79 g/I; p < 0.05) was decreased, respectively, in Type 2 diabetic patients compared with those of the controls. There were no significant differences in the concentrations of plasma thiol (0.0064 ± 0.001 vs. 0.0068 ± 0.001 µimole/mg protein), erythrocyte glutathione (2.54 ± 0.57 vs. 2.65 ± 0.56 mg/g Hb), plasma ceruloplasmin (548 ± 107.30 vs. 609 ± 93.34 mg/l) between the patients and the controls. These changes observed in diabetic patients contribute to the imbalance in the redox status of the plasma. We attribute this imbalance to oxidative protein damage in Type 2 diabetic patients clinically free of complications.