The purpose of this study is to determine the efficacy and safety of chronic treatment with inhaled dry powder mannitol in subjects with cystic fibrosis. Previous studies have demonstrated an improvement in lung function related to small airways obstruction and a significant improvement in respiratory symptoms and quality of life after a 2 week treatment with mannitol. This current study seeks to support these early findings and to extend the evidence to support its use as a mucoactive therapy in cystic fibrosis. In particular, the hypothesis that enhanced mucus clearance will improve the lung function and clinical presentation in this population, will be investigated. We also hypothesize that enhanced mucociliary clearance will result in a sustained reduction in mucus load, thus providing less opportunity for bacteria to proliferate, affording a reduction in antibiotic use and hospitalizations. The initial 6 month blinded phase will be followed with an additional 6 months of open label treatment.