Many cystic fibrosis patients die of lung failure caused by repeated lung infections from thick, sticky mucus. Past studies have shown Bronchitol inhalation may help to facilitate the clearance of mucus by altering its rheology and replenishing the airway surface liquid layer in these patients, thereby enhancing the shift of stagnant mucus from the lungs. The study aim is to determine the optimal dose of mannitol to generate clinical improvement in patients with cystic fibrosis.