The imbalance between oxygen free radicals and antioxidants, as defense mechanisms against free radicals, is called oxidative stress. Free radicals are reactive molecules with multiple effects on basic biomolecules such as cell membrane lipids, proteins and DNA. They are produced from extrinsic and intrinsic sources, mainly from cells involved in inflammatory procedures. Oxidative stress is involved in several respiratory diseases including bronchial asthma. Several studies regarding peripheral blood, bronchoalveolar lavage, bronchial mucosa cells, exhaled air and condensate, even urine, have shown increased free radicals production in asthma with coexistent disturbance in antioxidant mechanisms, as well as participation of free radicals in the pathogenesis and pathophysiology of asthma. There is also evidence that in some cases oxidative stress might be related to inflammation and severity of the disease. Decreased dietary antioxidant intake has been proposed as one of the factors that contribute to the increased incidence of asthma. However, research on extrinsic antioxidants administration has shown conflicting data, although administration of synthetic antioxidants in experimental animals showed more promising results. As the researchers are still interested in oxidative stress, more answers about the significance of oxidative stress and the significance of antioxidants administration in the therapy of asthma are expected. Pneumon 2006; 19(4):311-323.