Crossover Trial Determining the Efficacy of Dry Powder Mannitol to Improve Lung Function in Subjects Aged 6-17 Years

Inclusion Criteria: The subject must: 1. Personally provide, or have a legal guardian provide written informed consent to participate in the trial, according to local regulations; 2. rhDNase and maintenance antibiotic use is allowed but treatment must have been established at least 3 months prior to screening. The subject must remain on rhDNase and / or maintenance antibiotics for the duration of the trial. The subject must not commence treatment with rhDNase or maintenance antibiotics during the trial; 3. Have a confirmed diagnosis of cystic fibrosis (sweat test result greater than or equal to 60 mEq/L chloride and/or genotyping showing two identifiable mutations consistent with a diagnosis of cystic fibrosis); 4. Be aged greater than or equal to 6 years and \< 18 years; 5. Have a percentage of predicted FEV1 of greater than or equal to 30% and less than or equal to 90% at Screening (Visit 0). Percentage of predicted FEV1 will be calculated using Wang for children aged \< 8 years, and using NHanes III for those greater than or equal to 8 years; and 6. Be able to perform all the techniques necessary to measure lung function. Exclusion Criteria: The subject must NOT: 1. Be using maintenance nebulised hypertonic saline; 2. Be considered "terminally ill"; eligible for lung transplantation, or have received a lung transplant previously; 3. Require home oxygen or assisted ventilation; 4. Have had an episode of massive haemoptysis defined as acute bleeding ≥240 ml in a 24-hour period and/or recurrent bleeding ≥100 ml/day over several days in the three-months prior to Screening (Visit 0); 5. Have a known intolerance to mannitol; 6. Be taking non-selective beta-blockers; 7. In the three months prior to Screening (Visit 0) have had a myocardial infarction; a cerebral vascular accident; major ocular, abdominal, chest or brain surgery; 8. Have a known cerebral, aortic or abdominal aneurysm; 9. Be currently participating in, or have participated in another investigative drug trial within four weeks of Screening (Visit 0); 10. Be pregnant or breastfeeding, or plan to become pregnant whilst in the trial; 11. For females of childbearing potential, be using an unreliable form of contraception, (at the discretion of the investigator); 12. Have any concomitant medical, psychiatric, or social condition that, in the Investigator's opinion, would put the subject at significant risk, may confound the results or may significantly interfere with the subject's participation in the trial; or 13. Have a "failed" or "incomplete" mannitol tolerance test (as described in Section 8.3.1.1).