The main objective of this trial is to evaluate the effect of a 24-week treatment with inhaled glutathione compared with control inhalations (normal saline) on pulmonary function in adult and pediatric CF patients. Secondary objectives are to determine the effects of inhaled glutathione on inflammatory variables, glutathione levels and free elastase in induced sputum and to evaluate the safety and tolerability of the 24-week treatment with inhaled GSH.