 Lung clearance index LCI has been shown to correlate well with structural pulmonary alterations seen on CT even in CF patients with well-preserved spirometry. In cystic fibrosis, a high LCI is associated with a worse feeling of illness assessed by the Cystic Fibrosis Questionnaire-Revised CFQ-R. School-aged CF patients with normal spirometry also have normal aerobic function as assessed by peak oxygen uptake VO2 measured during symptom-limited incremental cardiopulmonary exercise test. However, the ability of LCI to predict ventilatory abnormalities appearing at exercise in CF patients has not been investigated. The investigators therefore aimed to compare physiological parameters at exercise between CF patients with elevated LCI i.e., LCI above the upper limit of normal ULN and patients with normal LCI, all with preserved spirometry. 