High-flow humidified oxygen (HFHO) is a respiratory therapy which allows the administration of an oxygen/air admixture via a nasal cannula at flows greater than 2 L/min. [1] The precise amount of oxygen delivered can be independently titrated to the oxygen flow with delivery of up to 100% oxygen attainable. In addition, the oxygen/air admixture is heated to a temperature of 34 °C and humidified to 'optimal humidity' with a water content of 44 mg/L. The benefits of HFHO therapy on the respiratory system appear to be numerous. [2,3] The high inspiratory flows result in reduced work of breathing, as well as washout of nasopharyngeal dead space. By warming and humidifying inspired gas, it firstly reduces the metabolic work of the patient, and secondly, it minimises the pulmonary broncho-constrictor response which is mediated by nasal muscarinic receptors. [4] In this way, both conductance as well as compliance in the lungs is improved. [5] Additionally, flows above 2 L/kg/min appear to provide some positive end-expiratory pressure (PEEP), estimated to be ~4 cmH 2 O. The amount of PEEP generated appears to be related to both the flow rate and size of the nasal cannula used. [6,7] At the time of embarking on our study, the use of HFHO therapy in infants with a diagnosis of bronchiolitis appeared to be a promising therapy, but its place in the routine management of these infants was not clear in the absence of data from high-quality RCT's. One RCT in infants with moderate bronchiolitis had shown that among infants given HFHO at 1 L/kg/min compared with those given 2 L nasal cannula oxygen, there was no difference between groups in terms of the time spent requiring oxygen. [8] This research aimed to test the hypothesis that there would be no difference in respiratory distress (as measured by the Modified TAL (M-TAL) score) in infants with bronchiolitis who have more severe disease (M-TAL score >6 and hypoxaemia in room air), when comparing standard oxygen therapy to HFHO therapy. The primary outcome assessed was the severity of respiratory distress (measured by the M-TAL score), and the secondary outcome