The study intervention is nebulised hypertonic (7%) saline (Active) or nebulised normal (0.9%) saline (Control) twice per day for 336 days. At a screening visit, subjects will complete quality of life questionnaires, be questioned regarding their medical history, undergo physical examination and spirometry, and will be requested to provide a sputum sample. The subject is then supervised taking their first dose to ensure the correct procedure is used and there are no adverse effects. The subject then commences taking the trial solution at home, and once a week completes a diary card to monitor factors such as respiratory tract infections and medication use. Subsequent visits are scheduled at Days 28, 84, 168, 252, 334, and 336, at which the same investigation are performed as at the screening visit.