The effects of intermittent positive-pressure hyperinflation on lung volumes, arterial blood gas tensions and respiratory muscle performance have been assessed in 10 patients with severe scoliosis (vital capacity 22–43% predicted). Hyperinflation was carried out 2–3 times a day for 5 min aiming to double the spontaneous tidal volume with each delivered breath. A volume-preset, time-cycled device was used in 6 patients and a pressure-cycled, patient-triggered machine in 4 cases. Progressive reduction in lung volume was reversed by the volume-preset device which increased the mean vital capacity from 29 to 33% predicted after 3 months (p