True segmental sagittal angle is defined as segmental sagittal angle in the sagittal plane of the vertebral segment itself, which is different from the apparent segmental sagittal angle measured on lateral radiograph since the vertebra is rotated. The purposes of this paper are to develop the method of calculating true segmental sagittal angle and to confirm their patterns in idiopathic thoracic scoliosis. A geometric model of vertebral segment was devised. The measured values on the A-P and lateral radiographs are the apparent segmental sagittal angle(a') and coronal angle( B'), and rotation of the vertebra. The relationship between them is induced as follows; The true segmental sagittal angle(B) corresponds to the B' when is zero. The angles of and can be obtained from the measured values of a', B' and . Then B can be obtained. Using this method, true segmental sagittal angles of 40 idiopathic thoracic scoliosis patients were calculated from the upper to the lower end vertebrae. The rotational angles of each segments were obtained by the Pedriolle method. There was little difference between the apparent and true segmental sagittal angles in the end vertebrae; 3° kyphosis vs. 2° kyphosis in the upper, and 1° kyphosis vs. 0° in the lower. However, the true segmental sagittal angle of apical vertebra was more lordotic than the apparent one by 6°. The sum of apparent segmental sagittal angles was 18° kyphosis and that of the true ones was 3° lordosis, while the normal value of the matched segments was 27 kyphosis. The difference between the apparent and true sagittal angles was more correlated with the apical rotation(r2=0.706) than with the Cobb angle(r2=0.303). In conclusion, a method of calculating the true segmental sagittal angle from the apparent segmental sagittal and coronal angle and rotation angle was developed. In idiopathic thoracic scoliosis, the true sagittal contour is more lordotic than the apparent one. This method may be useful in evaluating the scoliosis curve pre-and postoper-atively.