Twenty-six subjects with thoracic scoliosis due to various causes have been investigated. They all performed a progressive exercise test under standardised conditions, and their maximum oxygen uptake (VO2 max), blood gases, and ventilatory and heart rate responses were observed. The VO2 max was significantly below normal and was proportional to the forced expiratory volume in one second (FEV1) and maximum exercise ventilation (VE max). Exercise was limited by ventilatory factors in 80% of the subjects who exercised maximally. VE max was diminished, but the dyspnoeic index was normal. The minute ventilation (VE) at any given VO2 was about 20% greater than normal. This hyperventilation taken with the low VE max explains why exercise is usually ventilation-limited in these subjects. Tidal volume (VT) increased linearly with ventilation until VT max was reached. The tidal volumes at any given ventilation were less than normal, but when expressed as a percentage of vital capacity (VC) they were above normal. The heart rate increased more rapidly than normal, but this appears to be a physiological result of the small muscle mass of these subjects and not a pathological response.