The regional distribution of lung function was determined in the erect and supine positions in fifteen young patients with thoracic restriction due to varying degrees of kyphoscoliosis. The relative distribution of aerating lung volume, ventilation, and perfusion was determined using xenon-133 in sixty-six zones (four to six per patient). Gamma emissions were detected by a rack of four to six stationary, collimated sodium iodide crystals facing the posterior thoracic wall. Lung volume and alveolar-arterial gas exchange were determined by standard methods. The degree of curvature was determined by the method of Cobb from roentgenograms of the spine in the erect and supine positions; there was a mean decrease in curvature of 11 degrees (5 to 17 degrees) in the supine position. In the supine position regional abnormalities in the distribution of function were less frequent and aerating lung volume was more evenly distributed. Zones of abnormal ventilation occurred primarily on the concave side of the curve and were fewer in the supine (twelve) than in the erect (twenty-three) position. In the erect position perfusion tended to be increased in the upper zones and decreased in the lower zones. In three patients decreased lower-zone perfusion persisted in the supine position. These findings suggest that increased curvature in the erect position compresses the lung particularly on the concave side of the curve accounting for decreased aerating volume. The observed relative hypoventilation or hyperventilation appears to be caused by the variable expansion of lung produced by the deformed thorax, possibly related to abnormal distribution of intrapleural pressures. The increased upper-zone perfusion in the erect position is due to the decreased vertical height of the lung caused by the curvature. The decreased lower-zone perfusion appears to be related to decreased retractile forces on the extra-alveolar vessels which therefore collapse. The data suggest that these decreased forces may lead to permanent changes in pulmonary vasculature. Improvement in these abnormalities in the supine position when the degree of curvature is less suggests that stabilization or correction of these curves should be beneficial for pulmonary function as well as for cosmetic reasons provided that rib excursion is not impaired and permanent changes in pulmonary vasculature have not occurred.