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Objective: To measure the nasal gas flow in infants treated with bubble continuous positive airway pressure (CPAP) and compare it with commonly used flows during nasal high flow (nHF) treatment., Design: This is a prospective, single-centre study. Bubble CPAP pressure was measured at the nasal prongs. Set gas flow was reduced until bubbling in the water chamber just ceased. Set gas flow without bubbling then approximated flow entering the infant's nose ('delivered flow')., Setting: Neonatal intensive care at The Royal Women's Hospital, Melbourne, Australia., Patients: Clinically stable preterm infants receiving bubble CPAP therapy., Main Outcome Measure: Delivered flow (L/min) when bubbling stopped at a range of clinically set CPAP pressures (cm H 2 O)., Results: Forty-four infants were studied, with a mean (SD) gestational age at birth of 28.4 (2.2) weeks and birth weight of 1154 (419) g. At the time of the study, infants had a median (IQR) age of 4.5 (2-12) days and a mean (SD) weight of 1205 (407) g. Delivered flow ranged from 0.5 to 9.0 L/min, and increased with higher set CPAP pressures (median 3.5 L/min at CPAP 5 cm H 2 O vs 6.3 L/min at CPAP 8 cm H 2 O) and heavier weights (median 3.5 L/min in infants <1000 g vs 6.5 L/min for infants >1500 g)., Conclusions: Nasal gas flows during bubble CPAP in preterm infants are similar to flows used during nHF and increase with higher set bubble CPAP pressures and in larger infants. Trial registration number ACTRN12619000197134., Competing Interests: Competing interests: CJM is a consultant for Fisher & Paykel Healthcare., (© Author(s) (or their employer(s)) 2021. No commercial re-use. See rights and permissions. Published by BMJ.)