An oxidant/antioxidant imbalance is thought to play an important role in the pathogenesis of chronic obstructive pulmonary disease (COPD). We hypothesized that antioxidant capacity reflected by erythrocyte glutathione peroxidase (GPx) and superoxide dismutase (SOD) activities, and serum levels of the lipid peroxidation product malondialdehyde (MDA), may be related to the severity of obstructive lung impairment in patients with COPD. Erythrocyte GPx and SOD activities and serum levels of MDA were measured in 120 consecutive male participants. Erythrocyte GPx activity and erythrocyte SOD activities were significantly lower, and serum MDA levels were significantly higher in patients with COPD compared to non COPD. Findings of the present study suggest that antioxidant capacity reflected by erythrocyte GPx activity and erythrocyte SOD and serum levels of the lipid peroxidation product MDA are linked to the severity of COPD.