Inclusion Criteria:

CF population

• Children known with CF were recruited from the outpatient clinics. CF disease was defined as a combination of typical clinical features (e.g.

persistent pulmonary problems, meconium ileus, failure to thrive, steatorrhoe) and an abnormal sweat test (Chloride > 60 mM). Uncontrolled CF was diagnosed by the paediatric pulmonologist based on a change in the presence or severity of respiratory symptoms in association with CF, and/or a decrease in lung function parameters compared to previous measurements during the last four weeks.

Control population

• Control children without lung disease were recruited from the outpatient clinic of the University Hospital Maastricht. The reasons of consultation were constipation and enuresis nocturna. All children completed the ‘International Study of Asthma and Allergies in Childhood (ISAAC) questionnaire to exclude children with any (history of) airway or allergy complaints, in order to exclude asthmatic disease

Exclusion Criteria:

For both study populations:

• Diseases that may interfere with the results of the study (e.g. upper airway infection, heart disease, anatomic abnormalities of the airways and other chronic inflammatory diseases, such as Crohns disease and rheumatoid arthritis)
• Mental retardation
• Inability to perform the EBC collection procedure
• Active smoking
• Use of the following medication: papaverin, sodium nitroprusside, angiotensin-converting enzyme (ACE) inhibitors, oxymetazoline, L-arginine, or nitric oxide synthase (NOS) inhibitors.