Total work of breathing was measured on 55 occasions on 26 babies with acute bronchiolitis using a respiratory jacket to measure tidal volume and a naso-gastric tube to record oesophageal pressure. Work of breathing was increased six-fold on average and on one occasion exceeded forty times the expected. Approximately 40% of the intra-thoracic pressure swing was positive relative to atmospheric. It was not possible to correlate the work of breathing measurements with detailed clinical assessments.