Reactive oxygen species may be generated by several inflammatory cells which participate in airway inflammation and their production may be increased in asthma. Oxygen metabolites may contribute to the epithelial damage which is characteristic of asthmatic airways and may activate cells such as mast cells in the airway mucosa. Reactive oxygen species may cause bronchoconstriction, mucus secretion, have effects on airway vasculature, and may increase airway responsiveness. The role of reactive oxygen species in airway disease has been largely neglected, but appears to be an important area for future study. It is also possible that antioxidant defenses may be defective in asthma. If reactive oxygen species participate in the inflammatory response in airway disease, then radical scavengers or antioxidants could play a useful role in therapy.