BackgroundPulmonary function tests are important in the diagnosis and follow-up of airway disease in cystic fibrosis (CF). Conventional spirometry for which repeated forced expiration maneuver are needed is considered as the main method. Impulse oscillometry (IOS) is a non-invasive method, which needs minimal cooperation. We performed a prospective cross-sectional study to determine the pulmonary function in CF children with IOS, and evaluate the IOS measurements during acute exacerbation. We hypothesized that IOS can detect lung function impairment in CF patients and detect changes during acute exacerbations.