Inßuence of scoliosis surgery on pulmonary changes and even upon thoracic deformity changes are still controversial. The purpose of the study was to determine thoracic volume (TV) changes in patients operated on by means of posterior and anterior surgery because of severe thoracic AIS. 50 patients, operated by þrst author randomly chosen from the period 1993–97 were selected. In 25 patients (21 girls, 4 boys) anterior instrumentation was used (group 1), and posterior instrumentation in other 25 patients (20 girls, 5 boys; group 2). TV calculation was performed basing on preoperative and postoperative plain x-rays, using a well known equation (second and third, independent author). The curves were 73û ± 12.4 pre op, and 19û ± 15 post op (group 1), and 75û ±13 pre op., 37û± 10 post op. (group2). Calculated TV for group 1 increased from 5234 ml to 6043 ml postoperatively (17% ± 16). In group 2, TV increased from 5155 to 5489 to 4,371 (6% ± 7). The correlation between the Cobb angle change and the thoracic volume change was poor (+0.2 for group 1, -0.4 for group 2). To determine the role of frontal, sagital and vertical thoracic diameters in TV increase, further correlation tests were performed. The best correlation was found between the frontal and vertical diameter increase in anterior instrumentation (r=0.62; 0.71), whereas the best correlation was found between TV and sagital parameters in posterior instrumentation (r=0.74). It is concluded that anterior instrumentation can increase TV more than posterior instrumentation.