Life expectancy in CF patients has greatly increased due to improved clinical care. While this is certainly beneficial to CF patients, it has made it more difficult to assess the effect of therapeutic interventions. Currently, FEV1 remains the primary outcome parameter for most clinical trials, but many CF patients have normal pulmonary function and the annual rate of decline is now less than 2 %. Therefore, additional parameters are needed that are more sensitive to define abnormalities in CF patients and that can be used in therapeutic trials.