Thoracic volume was calculated in 50 adolescent patients operated on for severe idiopathic thoracic scoliosis. In 25, anterior instrumentation was used (group 1), and posterior instrumentation in the other 25 patients (group 2). Calculation of thoracic volume was made from measurements of pre-operative and post-operative radiographs. The mean spinal curvature in group 1 was 73+/-12.4 degrees before the operation, and 19+/-15 degrees after the operation, and in group 2 the curvature was 75+/-13 degrees before the operation and 37+/-10 degrees after the operation. The calculated thoracic volume in the group with anterior instrumentation increased from 5234 ml pre-operatively to 6043 ml post-operatively, while with posterior instrumentation it increased from 5155 ml to 5489 ml. The correlation between the change in the Cobb angle and the thoracic volume change was poor for both groups. To determine the role in the thoracic volume increase of the frontal, sagittal and vertical thoracic diameters, further correlation tests were made between these and the thoracic volume increase in each diameter. The best correlation was found between the frontal and vertical increase of diameters in group 1, whereas in group 2 the best correlation was found between the volume increase and the sagittal parameters.