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.

Inclusion Criteria:

• Diagnosis of cystic fibrosis (sweat test/genotype)
• 7 years or older
• FEV1 between 40% and 90% of predicted for height, age and gender.
• Able to perform acceptable-quality spirometry
• Clinically stable in the week up to study entry
• No additional antibiotics or additional oral steroids for a period of 14 days before study entry (routine antibiotics permitted)

Exclusion Criteria

• Currently active asthma
• Subjects colonized with Burkholderia cepacia or MRSA
• Considered "terminally ill" or listed for transplantation
• Requiring home oxygen or assisted ventilation
• Concurrent illness that in the investigators opinion may contribute to an increased and unacceptable risk if the subject was enrolled in the study (e.g. significant varicies, portal hypertension, cor pulmonale)
• Significant episode of haemoptysis (>60 mLs) in the previous 12 months
• Heart attack or stroke in last 3 months
• Known aortic or cerebral aneurysm
• Subjects who are breast feeding or pregnant.
• At risk females unwilling to use appropriate contraception to prevent pregnancy during the course of the study
• Subjects who have participated in another investigative drug study parallel to, or within 4 weeks of study entry.
• Known intolerance to mannitol or unable to take any form of bronchodilator medications.
• Uncontrolled hypertension, systolic BP > 200 or diastolic BP> than 100
• Concurrent use of beta blocker medication
• Concurrent use of hypertonic saline

Canada:

• Concurrent use of other pharmacological mucolytic agents other than Pulmozyme

Argentina:

• Concurrent use of other pharmacological mucolytic agents including Pulmozyme