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 less than 0.05) with improvement maintained after 9 months. This machine offered a significant advantage over the pressure-cycled device which at maximum or near maximum inflation pressures produced less hyperinflation and had no beneficial effect on lung volumes. The improvement in vital capacity was correlated with the degree of hyperinflation (r = 0.65; p less than 0.05). The effect of hyperinflation on arterial blood gas tensions was transient and not accompanied by an increase in accessible alveolar volume.