The current dream in CF research is to discover safe drugs that correct the basic defect and prevent lung disease, allowing patients without significant lung damage to live nearly normal lives with a dramatic increase in life expectancy and without the burden of current treatment. The compound VX-770 (Ivacaftor Ò) is hoped to be the first milestone along this way. Progression of lung disease is now so gradual in many centres that sensitive indicators of early lung disease (small airways disease) are critically needed to assess the effects of such new treatments. In this context, assessment of ventilation inhomogeneity by the measurement called Lung clearance index (LCI) seems to be the most promising tool. However, to get approval by health authorities, new measures used in drug evaluation need to fulfill strict criteria. For LCI, the investigators still need to prove its long term significance: How well does the LCI measurement predict the long term lung disease course? Therefore, in this study the investigators want to measure LCI at baseline in a large patient cohort and establish how well it predicts the patients' disease course over the next 2 years.

Patients with CF ( 6 years and older and FEV1 above 40% predicted) will be invited to participate in this prospective non interventional study in 2 CF centers ( UCL and UZ Leuven). Jointly we take care of more than 40 % of the Belgian CF population. Median FEV1 of children (6 - <18 y, n >120) from these CTN centers is high ( around 100% pred), defining a study group of great interest in the current context.

To assess the predictive value of LCI over 2 years, the time to next pulmonary exacerbation will be evaluated in the entire cohort. To assess the predictive value of LCI over 2 years, the time to next pulmonary exacerbation will be evaluated in the entire cohort.

Repeat LCI measurements will be done at subsequent patient visits.

2 devices measuring the LCI will be tested and compared : Ecomedics and NDD.

LCI and spirometry will be measured in 120 healthy Belgian children and young adults. These subjects must be free of respiratory symptoms for at least two weeks and will not have any chronic or recurrent chest problem.

Inclusion Criteria:

• FEV1(% predicted) > 40%

Exclusion Criteria:

• FEV1(% predicted) < 40%
• Exacerbation during baseline measurement

Patients with Cystic Fibrosis ( 6 years and older and FEV1 above 40% predicted) will be invited to participate in this prospective non interventional study in 2 CF centers (UCL and UZ Leuven)