Oxidants play a major role in mediating a variety of human disorders. The sources of oxidants may vary. They can be released by activated inflammatory cells or derive from the environment (such as pollution or tobacco smoke). In addition to acute, usually reversible reactions, numerous pulmonary diseases are associated with long lasting oxidant stress (adult respiratory distress syndrome, drug induced lung lesions, chronic obstructive pulmonary diseases, asthma, emphysema, interstitial fibrosis). Therapeutic action aimed at increasing anti-oxidant defense mechanisms is still a clinical challenge. Administration of N-acetylcysteine by the oral route is a simple and efficacious means of correcting oxidant-antioxidant imbalance.