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Background: Cystic fibrosis (CF) is commonly characterised by thick respiratory mucus. From diagnosis, people with CF are prescribed daily physiotherapy, including airway clearance techniques (ACTs). ACTs consume a large proportion of treatment time, yet the efficacy and effectiveness of ACTs are poorly understood. This study aimed to evaluate associations between the quality and quantity of ACTs and lung function in children and young people with CF., Methods: Project Fizzyo, a longitudinal observational cohort study in the UK, used remote monitoring with electronic pressure sensors attached to four different commercial ACT devices to record real-time, breath-by-breath pressure data during usual ACTs undertaken at home over 16 months in 145 children. ACTs were categorised either as conformant or not with current ACT recommendations based on breath pressure and length measurements, or as missed treatments if not recorded. Daily, weekly and monthly associations between ACT category and lung function were investigated using linear mixed effects regression models adjusting for clinical confounders., Results: After exclusions, 45 224 ACT treatments (135 individuals) and 21 069 days without treatments (141 individuals) were analysed. The mean±sd age of participants was 10.2±2.9 years. Conformant ACTs (21%) had significantly higher forced expiratory volume in 1 s (FEV 1 ) (mean effect size 0.23 (95% CI 0.19-0.27) FEV 1 % pred per treatment) than non-conformant (79%) or missed treatments. There was no benefit from non-conformant or missed treatments and no significant difference in FEV 1 between them (mean effect size 0.02 (95% CI -0.01-0.05) FEV 1 % pred per treatment)., Conclusions: ACTs are beneficial when performed as recommended, but most people use techniques that do not improve lung function. Work is needed to monitor and improve ACT quality and to increase the proportion of people doing effective airway clearance at home., Competing Interests: Conflicts of interest: G. Davies reports personal UKRI fellowship, UKRI and NIHR grants, speaker honoraria from Chiesi Ltd, and speaker honoraria, consulting fees and participation on a board from Vertex Pharmaceuticals, outside the submitted work. S. Stanojevic reports consulting fees from Chiesi Ltd, speaker fees from Vyaire Medical, participation on a board for Ndd technologies, and leadership role from the European Respiratory Society and the American Thoracic Society, outside the submitted work. The remaining authors have no potential competing interests to disclose., (Copyright ©The authors 2023.)