Objective: To compare the effects of nasal continuous positive airway pressure (nCPAP) and conventional oxygen therapy on the clinical signs of respiratory distress and the respiratory muscle workload in acute viral bronchiolitis.Design: Prospective, randomized, monocentric study carried out in the pediatric intensive care unit (PICU) of a university hospital.Patients: Infants <6 months old, admitted to the PICU with severe respiratory syncytial virus bronchiolitis.Intervention: The patients were randomized into two groups for 6 hr. The nCPAP group (n = 10) received 6 cmH(2)O pressure support delivered by a jet flow generator and the control group (n = 9) received an air/oxygen mixture from a heated humidifier. Respiratory distress was assessed by the modified Wood's clinical asthma score (m-WCAS), and inspiratory muscle work was evaluated by calculating the pressure-time product per breath (PTP(insp) /breath) and per minute (PTP(insp) /min) from the esophageal pressure (Pes) recordings.Measurements and main results: Compared with control condition, nCPAP decreased m-WCAS [-2.4 (1.05) vs. -0.5 (1.3), P = 0.03], PTPes(insp)/breath [-9.7 (5.7) vs. -1.4 (8.2), P = 0.04], PTPes(insp) /min [-666 (402) vs. -116 (352), P = 0.015], and FiO(2) [-7 (10) vs. +5 (15), P = 0.05]. Significant worsening of m-WCAS was only observed in the control group (4/9 vs. 0/10, P = 0.03).Conclusions: nCPAP rapidly decreased inspiratory work in young infants with acute bronchiolitis. Improvement in the respiratory distress score at 6 hr was proportional to the initial clinical severity, suggesting the importance of rapid nCPAP initiation in the more severe forms of the disease.