Primary objective of the study is to investigate whether a 12 months treatment with inhaled GSH can improve airway obstruction in CF patients. Secondary objectives include the effects of GSH therapy on exercise capacity, body mass index (BMI), respiratory symptoms, quality of life, frequency of pulmonary exacerbations, hospital admissions, and antibiotic administration. Moreover the study will evaluate the effect of GSH therapy on markers of oxidative stress in exhaled breath condensate (EBC) and in serum, and on inflammatory markers on brushed nasal epithelial cells.