Lipid peroxidation-antioxidants were studied in condensed expired air of patients with chronic bronchitis. Under conditions of bronchopulmonary system impairment considerable activation of lipid peroxidation occurred in surface-active lung layers. These processes were elevated during development of the bronchial obstruction syndrome. Alterations of lipid peroxidation were amplified in patients with chronic bronchitis as functional impairments of the respiratory system increased. The antiradical defense system of lung surfactant was found to correlate inversely with the disease duration and severity of respiratory insufficiency.