The widespread neonatal detection of cystic fibrosis in France since 2002 permits to treat children from birth. New treatments used for young children involve to assess efficacy criteria specific to this population. Standard respiratory function criteria for older children and adults is forced expiratory volume/second.

This technique is not suited for preschool aged children (3 to 6 years old) because they are too old to be sedated and too young and immature to be able to make forced expiration technique that are correct, reproducible and prolonged during more than 1 second.

For preschool aged children, in order to assess distal damage and her consequence, the evaluations are: airway resistance by debit interruption technic (Rint), plethysmographic measure of specific resistance (sRaw), functional residual capacity by Helium dilution technique (CRF He), arterial blood gas measurement, pulmonary clearance index.

All these methods have a better success rate and can be used in alternative or with forced spirometry. However, each of them gives only a part of information on airway and lung damage of detected children. It is necessary to combine them for a better information on overall respiratory damage.

In France, each respiratory function test laboratory uses one or any of these methods in addition to flow-volume curve, in function of his practices and his equipment.

So, respiratory function test of preschool aged children is going to diversify more and more to the detriment of an homogeneity of practices between different centers.

A referent population during a longitudinal multicenter monitoring on large cohorts that describe the evolution of pulmonary function, obtained by a standardized methodology is necessary to assess the efficacy of any new treatment. And, with the homogenization of care of children detected of cystic fibrosis in different centers, the description of natural evolution of pulmonary function by a standardized methodology will improve the discriminative power of measure of respiratory function to assess the presence of a worsening in preschool-aged children.

Inclusion Criteria:

• Patient suffering from cystic fibrosis
• Height between 90 et 130cm
• No respiratory exacerbation since 4 weeks
• Benefit from an insurance disease regime

Exclusion Criteria:

• Law-protected patient
• Patient's parent don't understand french language
• Opposition to participation