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Introduction: Exercise intolerance is common in chronic airway diseases (CAD), but its mechanisms are still poorly understood. The aim of this study was to evaluate exercise capacity and its association with lung function, ventilatory limitation, and ventilatory efficiency in children and adolescents with cystic fibrosis (CF) and asthma when compared to healthy controls., Methods: Cross-sectional study including patients with mild-to-moderate asthma, CF and healthy children and adolescents. Anthropometric data, lung function (spirometry) and exercise capacity (cardiopulmonary exercise testing) were evaluated. Primary outcomes were peak oxygen consumption (VO 2 peak), forced expiratory volume in 1 s (FEV 1 ), breathing reserve (BR), ventilatory equivalent for oxygen consumption (V E /VO 2 ) and for carbon dioxide production (V E /VCO 2 ), both at the ventilatory threshold (VT 1 ) and peak exercise., Results: Mean age of 147 patients included was 11.8 ± 3.0 years. There were differences between asthmatics and CF children when compared to their healthy peers for anthropometric and lung function measurements. Asthmatics showed lower VO 2 peak when compared to both healthy and CF subjects, although no differences were found between healthy and CF patients. A lower BR was found when CF patients were compared to both healthy and asthmatic. Both CF and asthmatic patients presented higher values for V E /VO 2 and V E /VCO 2 at VT 1 when compared to healthy individuals. For both V E /VO 2 and V E /VCO 2 at peak exercise CF patients presented higher values when compared to their healthy peers., Conclusion: Patients with CF achieved good exercise capacity despite low ventilatory efficiency, low BR, and reduced lung function. However, asthmatics reported reduced cardiorespiratory capacity and normal ventilatory efficiency at peak exercise. These results demonstrate differences in the mechanisms of ventilatory limitation to maximum exercise testing in children and adolescents with CAD., (© 2023 The Authors. Pediatric Pulmonology published by Wiley Periodicals LLC.)