Cystic fibrosis (CF) results in chronic obstructive pulmonary disease with progressive increases in areas of the lung with poor gas exchange (dead space). The pulmonary response to exercise is characterized by a higher than expected minute ventilation (V̇E). In some patients the elevated V̇Eis inadequate to overcome the large dead space, resulting in decreases in arterial oxygen and/or increases in arterial carbon dioxide levels during exercise. Chronic lung hyperinflation in patients with severe disease limits the ability to increase tidal volume, and therefore V̇E, during exercise. Most patients can exercise without decreases in arterial oxygen and should be encouraged to participate in regular activity. Oxygen supplementation during exercise allows many patients with severe lung dysfunction to participate in some form of activity. Regular exercise has been shown to be of some benefit and should be encouraged on an individual basis in patients with CF.