The pathogenesis of CF lung disease may start in infancy. Therefore, it is important to monitor the early stages of its progress. The Exhalyzer D is the first commercially available device designed to measure lung ventilation inhomogeneity at any age. This study was conducted to assess the performance and feasibility of using the Exhalyzer D in a paediatric CF clinic. A total of 91 subjects were recruited (23 controls, and 68 patients with CF). The majority of CF patients (79%) and controls (78%) completed at least two successful washouts. A strong linear correlation was noted between LCI and FEV1. Children with CF under six years of age struggled to perform the washout in a technically correct manner. A clear learning effect was observed, with improved technique and shorter testing times on repeated visits.