Pulmonary function and gas exchange at rest and during and after standard exercise on bicycle ergometer were studied in 49 girls, aged 10 to 16 years (mean, 13.7), with untreated idiopathic scoliosis, before and with Boston thoracic brace, and after treatment for 3 weeks, reducing the thoracic curve angle from a mean of 32.4 degrees (range, 19-60 degrees) to 17.3 degrees (range, 8-33 degrees). The bracing led to marked decrease in lung volumes, below functions, and CO transfer factor, moderate rise in resting O2 intake, CO2 output and ventilation, and marked rise in ventilatory and O2 cost of exercise, probably due to increased dead space ventilation and O2 cost of breathing. However, repeated studies after 1 day without brace, 6 months after onset of bracing, showed no persistent deterioration of pulmonary function or disturbances of gas exchange, as compared with the pre-bracing state.