 The purpose of this study is to examine the efficacy and safety of 26 weeks treatment with inhaled mannitol in subjects with cystic fibrosis. Previous studies have demonstrated improvements in lung function, mucociliary clearance, changes in physical properties of mucus, 24 hour sputum weight and quality of life. The results of this study are to further investigate and confirm these findings in addition to examine the effect on antibiotic use and chest infections. It is hypothesised that inhaled mannitol will have beneficial effects compared to a control treatment. An open label phase of 26 weeks duration will follow the blinded 26 week phase. During the open label phase all subjects will receive active treatment. 