Free radicals are usually reactive species, characterized by the possession of one or more unpaired electrons. This molleculles are involved in the pathogenesis of various respiratory diseases. Free radicals have been evaluated in the bronchoalveolar lavage to understand the importance of its role in lung cancer, adult respiratory distress syndrome, interstitial pulmonary diseases, tobacco induced pulmonary damage, hyperoxia and drug-induced lung toxicity. There are evidences suggesting that these agents play an important role in lung injury. These findings could lead the development of new, and more effective, therapeutic agents in some respiratory diseases.