Mannitol Dose Response Study in 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