Mechanisms causing the reduction in lung capacity commonly found in adolescents with idiopathic scoliosis (IS) have not been understood. In 29 patients with typical thoracic curvatures of mild to moderate degree (less than 60 degrees), total lung capacity (TLC) was a mean 75 +/- 13% (SD) of predicted. The patients could generate only -70 +/- 26 cm H2O (SD) maximal inspiratory airway pressure at function residual capacity, as compared with -102 +/- 28 cm H2O in 21 normal control subjects (p less than 0.001). Studies of lung mechanics in 15 of the patients showed that maximal transpulmonary pressure at TLC was also reduced. Static pressure volume curves were shifted to the right, and both static and dynamic lung compliance were significantly reduced. Although both upstream conductance per TLC and anatomic dead space per TLC were abnormally high, relationships between maximal expiratory flow and static lung recoil were appropriate for age, indicating a normal growth of airway dimensions. From results of the single-breath nitrogen washout procedure, amounts of trapped nitrogen were also normal, indicating that the low lung compliance is not caused by airway closure. After a 5-min period of positive pressure (25 cm H2O) breathing, dynamic compliance increased by a mean of 34% in subjects with low TLC, and by a significantly smaller (p less than 0.05) mean increase of 14% in subjects with normal TLC. The 15 patients were restudied 1 yr after corrective surgery by the Harrington procedure.(ABSTRACT TRUNCATED AT 250 WORDS)