To determine the optimal level of nasal continuous positive airway pressure (nCPAP) in infants with severe hypercapnic viral bronchiolitis as assessed by the maximal unloading of the respiratory muscles and improvement of breathing pattern and gas exchange. A prospective physiological study in a tertiary paediatric intensive care unit (PICU). Breathing pattern, gas exchange, intrinsic end expiratory pressure (PEEPi) and respiratory muscle effort were measured in ten infants with severe hypercapnic viral bronchiolitis during spontaneous breathing (SB) and three increasing levels of nCPAP. During SB, median PEEPi was 6 cmH2O (range 3.9–9.2 cmH2O), median respiratory rate was 78 breaths/min (range 41–96), median inspiratory time/total duty cycle (T i/T tot) was 0.45 (range 0.40–0.48) and transcutaneous carbon dioxide pressure (P tcCO2) was 61.5 mmHg (range 50–78). In all the infants, an nCPAP level of 7 cmH2O was associated with the greatest reduction in respiratory effort with a mean reduction in oesophageal and diaphragmatic pressure swings of 48 and 46%, respectively, and of the oesophageal and diaphragmatic pressure time product of 49 and 56%, respectively. During nCPAP, median respiratory rate decreased to 56 breaths/min (range 39–108, p < 0.05), median T i/T tot decreased to 0.40 (range 0.34–0.44, p < 0.50) and P tcCO2 decreased to 49 mmHg (range 35–65, p < 0.05). Only one infant with associated bacterial pneumonia required intubation and all the infants were discharged alive from the PICU after a median stay of 5.5 (range 3–27 days). In infants with hypercapnic respiratory failure due to acute viral bronchiolitis, an nCPAP level of 7 cmH2O is associated with the greatest unloading of the respiratory muscles and improvement of breathing pattern, as well as a favourable short-term clinical outcome.