Purpose: The high flow nasal cannula (HFNC) has recently been proposed to support infants with respiratory syncytial virus (RSV)-related respiratory distress. However, in this disease, no physiologic data are currently available on the effects of this device. We assessed the capacity of HFNC to generate positive airway pressure, as well as the resulting effects on breathing pattern and respiratory effort. Methods: Twenty-one infants less than 6 months old with acute RSV bron-chiolitis were studied prospectively in the pediatric intensive care unit of a university hospital. Pharyngeal pressure (PP) and esophageal pressure (Pes) were measured simultaneously at four increasing flows of 1, 4, 6 and 7 L/min delivered through HFNC. Results: The PP was correlated with flow rate (r = 0.65, p B 0.0001), reaching mean and end-expiratory values of, respectively, 4 (95% CI 3-5) cmH 2 O and 6.5 (95% CI 5-8) cmH 2 O at 7 L/min. A flow C2 L/kg/min was associated with the generation of a mean pha-ryngeal pressure C4 cmH 2 O with a sensitivity of 67 %, a specificity of 96 %, a positive predictive value of 75 %, and a negative predictive value of 94.5%. Only flows C6 L/min provided positive PP throughout the respiratory cycle. From baseline to maximal flow rate, breathing frequency (p \ 0.01), T i /T tot (p \ 0.05), Pes swing (p \ 0.05) and PTPes insp / min (p \ 0.01), an index of respiratory effort, were reduced. Conclusions: HFNC with a flow rate equal to or above 2 L/kg/min generated a clinically relevant PP, with improved breathing pattern and rapid unloading of respiratory muscles , in young infants with acute RSV bronchiolitis.