Chronic obstructive pulmonary disease (COPD) is one of the most common chronic diseases and a major cause of morbidity and mortality. An imbalance between oxidants and antioxidants (oxidative stress) has been proposed as a critical event in the pathogenesis of COPD. The increased oxidative stress in patients with COPD is the result of exogenous oxidants namely pollutants and cigarette smoke as well as endogenous oxidant production during inflammation. The aim of the present study was to clarify the hypothesis about the presence of an imbalance between oxidants and the antioxidant defences associated to COPD. In this study, we evaluated a biomarker of oxidative stress (malondialdehyde, a lipid peroxidation derived product) and non-enzymatic antioxidants (vitamin C and the sulphydryl groups) in COPD patients and healthy controls. The marker of oxidative stress was found to be significantly (p < 0.001) higher in COPD patients when compared with control group. No age dependent changes in the plasma levels of lipid peroxidation products were found. COPD patients had a significant (p < 0.001) decrease in antioxidant status as compared with the control group. Our results show that oxidative stress is an important pathophysiologic change in COPD. © 2012 Sociedade Portuguesa de Pneumologia. Published by Elsevier Espana, S.L. All rights reserved.