There is an increasing interest in the concept that oxidant/antioxidant imbalance plays a role in the pathogenesis of chronic obstructive pulmonary disease (COPD). However, most of the studies are concentrated on the local antioxidant/oxidant balance. In this study, we investigated the oxidant/antioxidant balance in systemic circulation of patients with COPD. Serum malonyldialdehyde (MDA), vitamin C and erythrocyte reduced glutathione (GSH) were determined in patients during acute exacerbation and during the stable phase of the disease, and compared with age- and sex-matched healthy controls. The levels of serum MDA, vitamin C and erythrocyte GSH were determined according to Yagi, Beutler and Bauer et al., respectively. Serum MDA levels were significantly higher in patients compared to controls, and during acute exacerbation compared to the stable phase. MDA levels in patients with acute exacerbation and in those in stable phase were also higher than in controls. We found significantly decreased levels of erythrocyte GSH and serum vitamin C in patients with acute exacerbation and stable COPD compared to controls. Although smoking caused an increase in oxidative stress in controls, the measured parameters were not affected by smoking in the patient group. In conclusion, there is a systemic oxidant/antioxidant imbalance in COPD, and this imbalance is probably independent of smoking.