COPD is a major cause of ill health and death in Australia with 40,000 hospital admissions, and a national cost of $898,000,000 annually. The gold standard treatment of COPD is steroids for inflammation, antibiotics for infection and bronchodilators and oxygen for respiratory failure. However, associated mucus hypersecretion is responsible for much of the inflammation and infection. The use of pharmaceutical agents to assist in the early clearance of the retained mucus has been limited, primarily because of lack of demonstrated effect. There has been a recent development of interest in pursuing new therapies for improving mucociliary clearance and several studies have demonstrated that clinical outcomes can be improved when osmotic agents such as mannitol are added to standard treatments. The purpose of this study is to conduct a pilot safety study in patients with an acute exacerbation of COPD to determine if it is safe to administer inhaled mannitol for facilitating mucus clearance.