Recently, increased oxidative stress and impaired antioxidant defense have been suggested as a contributory factor for initiation and progression of complications in diabetes. Although glutathione (GSH) and the enzymes included by glutathione redox cycle have an important role for protection of cells against free radical-mediated damage, they may be susceptible to oxidation themselves. We examined the susceptibility of the GSH pathway to oxidation and inactivation in subjects with well-controlled and poorly controlled insulin-dependent diabetes mellitus (IDDM) versus controls and the effect of glycemic control on this susceptibility. Red blood cells (RBCs) were isolated, RBC level of GSH, activity of glutathione peroxidase (G-Px), and glutathione reductase (G-Red) were measured at the baseline and after a 2-hour incubation with hydrogen peroxide. Significant decreases were observed in the GSH level and in the activity of GSH peroxidase and GSH reductase in all the groups after the incubation with hydrogen peroxide. Maximum decrease was observed in the poorly controlled diabetic group for all parameters. This result indicates that the GSH pathway is susceptible to oxidation; and this susceptibility increases in poorly controlled diabetics. Therefore, insufficient antioxidant defense by the GSH pathway may be one of the factors responsible for development of complications in patients with IDDM.