The purpose of this study was to determine the influence of morphologic alteration on lung function in children with pectus excavatum (PE) and to establish the usefulness of chest radiography to predict the imbalance of pulmonary perfusion and lung volume. Chest radiography, pulmonary perfusion scintigraphy, and computed tomography (CT) for calculation of each lung volume were performed in 38 children with PE. To assess the relationship between position change of the mediastinum and lung, the following indices were calculated: (1) vertebral index (VI); the severity of sternal depression on the lateral chest radiograph, (2) left displacement index (LDI); the ratio between the left border of the mediastinum and the left border of the thorax to the transverse thoracic dimension on posterioanterior chest radiography; (3) left-to-right count ratio for the lung scintigraphy (Ls/Rs), and (4) the left-to-right thorax volume ratio from the CT scan (Lv/Rv). Compared to children without PE, VI was significantly higher, and LDI was lower in patients with PE. Pulmonary perfusion scintigraphy and CT showed that left pulmonary perfusion and lung volume were significantly lower than in the right lung in PE. LDI showed a close correlation with Ls/Rs (R = 0.443, P = 0.005) and Lv/Rv (R = 0.703, P < 0.001). Left displacement of the mediastinum within the closed thoracic cavity directly imposes constraints on the left lung, resulting in reductions of perfusion and lung volume that likely determine the physiologic severity of lung function in PE. Posterioanterior chest radiography might be useful in predicting the severity of PE.