Cystic fibrosis (CF) is the most common, inherited, and life-shortening disease among the white population. Males and females are equally affected. However, Ireland with a rate of one in 1600 births has the highest incidence of CF worldwide. CF patients with the highest aerobic fitness levels have a survival rate of 83% at 8 years of age compared to 51% at the same age in patients with the lowest fitness levels. Arresting the decline in aerobic fitness could therefore potentially increase the quality and length of life of CF patients. We have preliminary data which shows that supplemental oxygen may attenuate the ventilatory limitation in CF patients during exercise and enhance both training intensity and duration. We plan to investigate the effect of using supplemental oxygen during acute and chronic exercise in patients with mild, moderate and severe CF.