Aims: The inspired oxygen concentration (FiO2) is an important criterion for assessing the severity of bronchiolitis. Oxygen delivery by nasal prongs is a measure of oxygen flow, but not FiO2. We aimed to determine whether FiO2 of oxygen delivered by nasal prongs could be predicted from nasal flow by relating arterial oxygen concentrations achieved with prongs to those achieved via head box in children with bronchiolitis. Methods: This is a pilot study conducted at a tertiary referral paediatric hospital. We studied hospitalised children less than 24 months old requiring supplemental oxygen because of bronchiolitis, an acute viral lower respiratory tract infection. Children admitted to the intensive care unit, and those with congenital cardiac disease or recent bronchodilator use were excluded. Subjects were studied in nasal prong, then head box oxygen. Arterial oxygen concentration was measured by a transcutaneous probe (tcPO2). Oxygen flows by nasal prongs and FiO2 by head box were adjusted to achieve similar tcPO2 readings. FiO2 values were plotted against oxygen flow rates based on matching tcPO2. Results: We recorded tcPO 2 across a satisfactory range of values in eight children. TcPO 2 increased with increasing FiO2 and nasal oxygen flow, but at variable rates between subjects. FiO2 increased with increasing nasal oxygen flow, but this relationship was highly variable. Conclusions: In this study, it was not possible to estimate FiO2 reliably from nasal oxygen flow rates in children with bronchiolitis. Nasal prong oxygen flow rates should be used with caution when assessing the severity of bronchiolitis in children.