It is presumed that early detection and treatment of the chronic lung diseases cystic fibrosis (CF) and primary ciliary dyskinesia (PCD) are crucial for improving the prognosis. Mucus stagnation and secondary infections and inflammation in the airways causes progressive lung damage from infancy in CF and PCD, which again causes early death in CF and reduced pulmonary function in PCD. Early detection of ongoing lung damage will facilitate early intervention with intensive treatment, enhance disease surveillance and further improve preventive treatment. This will improve quality of life for patients with both CF and PCD and likely improve survival in CF.Pulmonary function is an important marker of prognosis and effect of treatment, but current pulmonary function tests do not register the earliest damages in the lungs in school children \u2013 and are inappliccable for infants and toddlers due to requirements for good coorporation. For this reason there is a demand for more sensitive methods that can be used in all age groups. Gas washout \u2013 multiple breath inert gas washout (MBW) \u2013 is a method that meets these criteria and seems promising for detection of minimal lung damage prior to debut of clinical symptoms and detection by other methods.MBW is for the first time in Denmark established in the Pediatric Pulmonary Service at Copenhagen University Hospital, where a recently established \u201dRespiratory Physiology Lab for Children\u201d offers a wide range of methods to detect lung disease in children.Aim:The aim of the Ph.D.-project is to investigate:Reliability of results conducted from MBW measurements under different standardized conditions (reproducibility), including establishment of a Danish reference material for children.Whether MBW can be used as a marker of early lung disease compared to conventional lung function methods in the chronic lung diseases CF and PCD.Whether MBW can be used as a relevant outcome to detect interventions in basic CF treatment.The Ph.D.-project will generate a new and more aggressive approach to surveillance and treatment of patients with CF and PCD with an overall aim to reduce morbidity and mortality and improve quality of life. Furthermore, it is expected that this project will generate new knowledge that can be used in the more common lung diseases asthma and COPD, which affects 10% and 15% of children and adults, respectively.