Restrictive pulmonary function abnormalities are reported in children and adolescents with idiopathic scoliosis. We hypothesized that spirometry alone, without more extensive testing, including the measurement of lung volumes, is inadequate in characterizing lung function in these children and may miss obstructive abnormalities including significant gas trapping. To examine this hypothesis, we reviewed the pulmonary function tests of 44 children (36 female, 8 male) between the ages of 10 and 18 years with idiopathic scoliosis prior to surgical correction. Spirometry, measurements of lung volumes with both plethysmographic and helium dilution techniques, and bronchodilator response were analyzed for evidence of reversible airway obstruction and gas trapping. Eighteen of 44 (41%) subjects had significant restriction. Only 3 (7%) subjects met standard criteria for airflow obstruction. However, 20 (46%) subjects had an elevated total gas volume by plethysmography-functional residual capacity by helium dilution ratio indicative of moderate or severe gas trapping, 10 (23%) subjects showed mild gas trapping, 8 (18%) subjects had a ratio suggestive of gas trapping, and only 6 (14%) subjects were normal. Additionally, significant improvement in airway mechanics was noted after bronchodilator administration. Specific conductance improved in all subjects, with a mean increase of 62% +/- 8.0 (p<0.001). The residual volume-total lung capacity ratio and total gas volume by plethysmography also decreased significantly (mean decrease, 22.5% +/- 3.0 and 15% +/- 1.0, respectively, p<0.001) in response to inhaled bronchodilators. In conclusion, although restrictive defects are commonly present in children with idiopathic scoliosis, significant gas trapping and responses to bronchodilators also commonly occur. These abnormalities may be missed without extensive pulmonary function testing.