Inclusion Criteria

• Have given written informed consent to participate in this trial in accordance with local regulations
• Have a confirmed diagnosis of cystic fibrosis (positive sweat chloride value ≥ 60 mEq/L) and/or genotype with two identifiable mutations consistent with CF, accompanied by one or more clinical features consistent with the CF phenotype)
• At least 18 years old
• Have FEV1 > 40 % and < 90% predicted (using NHanes III [1])
• Able to perform all the techniques necessary to measure lung function
• Adherent with maintenance therapies (antibiotics and or rhDNase), if used, for at least 80% of the time in the two weeks prior to visit 1
• If rhDNase and/or maintenance antibiotic are being used treatment must have been established at least 1 month prior to screening visit 0.
• The subject should remain on the rhDNase and / or maintenance antibiotics for the duration of the trial.
• The subject should not start treatment with rhDNase or maintenance antibiotics during this trial

Exclusion Criteria

• Investigators, site personnel directly affiliated with this trial, or their immediate families. Immediate family is defined as a spouse, parent, child or sibling, whether biologically or legally adopted
• Considered "terminally ill" or eligible for lung transplantation
• Have had a lung transplant
• Used a maintenance nebulized hypertonic saline in the 2 weeks prior to visit 1
• Have had a significant episode of hemoptysis (> 60 mL) in the three months prior to visit 0
• Have had a myocardial infarction in the three months prior to visit 0
• Have had a cerebral vascular accident in the three months prior to visit 0
• Have had major ocular surgery in the three months prior to visit 0
• Have had major abdominal, chest or brain surgery in the three months prior to visit 0
• Have a known cerebral, aortic or abdominal aneurysm
• Are breast feeding or pregnant, or plan to become pregnant while in the trial
• Are using an unreliable form of contraception (female subjects at risk of pregnancy only)
• Are participating in another investigative drug trial, parallel to, or within 4 weeks of screening visit 0
• Have a known allergy to mannitol
• Are using non-selective oral beta blockers
• Have uncontrolled hypertension -i.e. systolic BP > 190 and / or diastolic BP > 100
• Have a condition or in a situation which in the Investigator's opinion may put the subject at significant risk, may confound results or may interfere significantly with the subject's participation in the trial
• Have a failed or incomplete MTT at trial entry