To compare the sitting balance capabilities of healthy young women with those of women with idiopathic scoliosis, we performed experiments using a system for evaluating dynamic sitting balance. The system comprised a three-dimensional position and orientation sensor for measuring the upper body joint angle, a stabilometer for measuring the center of pressure (COP) trajectory and seat reaction force, and a six-degree-of-freedom platform for producing seat perturbation. The seat reaction force and joint angle of the upper body were used to calculate the joint torque of the 5th lumbar and 12th thoracic vertebra. During the experiments, 13 control subjects and 21 women with idiopathic scoliosis tried to maintain a sitting posture relative to the seat perturbation (0.25 Hz and 0.5 Hz, 5 deg amplitude). For seat perturbation of 0.5 Hz, the root mean square, mean velocity along the trajectory, and excursion area (COP parameters), and the joint torque of the 12th thoracic vertebra in women with idiopathic scoliosis were significantly higher than those in the control subjects. Results suggest that the dynamic sitting balance ability of women with idiopathic scoliosis is inferior and that a large physical load is necessary for them to maintain a sitting posture.