Several reactive oxygen intermediates are generated in biological processes involved in cellular respiration and during the respiratory burst of phagocytic cells. Exogenous insult of free radicals to respiratory tract may derive from polluting environmental agents, cigarette smoke, drugs, toxic compounds, and hyperoxia. Oxidizing radicals cause damage to proteins, lipids, carbohydrates, enzymes, nucleic acids, and other biological constituents. They are counterbalanced by different defensive mechanisms present in the body, whose action may be enhanced by exogenous antioxidant supply. Oxidants may have a role in patients with COPD, since the majority of these subjects are heavy smokers and show increased amount of highly activated phagocytes in their respiratory tract. This determines inactivation of the antiprotease system, increase in elastase production, and impairment of the connective tissue repair, so leading to structural changes that characterize chronic obstructive pulmonary disease.