Oxidant generation is part of the normal metabolism of many types of cells. To protect itself against exposure to noxious oxidants the lung has a well-developed antioxidant system. When an imbalance occurs between oxidants and antioxidants in favor of oxidants, oxidative stress is said to occur. Many experimental and clinical data suggest that an imbalance between oxidants and antioxidants (enzymatic and/or nonenzymatic) play a role in the pathogenesis of several respiratory disorders. Here we will review the nonenzymatic antioxidant systems of the lower airways. Most of the clinical data currently available refer to the glutathione system. Some studies have been recently performed on other nonenzymatic antioxidants such as vitamin C and vitamin A.