Growth hormone (GH) therapy is now widely given to Prader–Willi syndrome (PWS) patients to encourage growth in body height and to prevent obesity. Scoliosis, one of the complications in this syndrome, is thought to be accelerated in parallel with a rapid increase in body height, especially during adolescence. To determine whether GH therapy aggravates scoliosis and to identify any factor which might predict the progression of scoliosis, we studied 35 (22 males and 13 female) PWS patients between the ages of 2–16 years on GH therapy whose scoliosis was followed with spinal X-rays every 6 months. Thirteen (37.1%) of 35 patients had scoliosis with a Cobb angle of over 10°. Scoliosis was unchanged in five patients (14.3%), became worse in six (17.1%) and improved in two (5.7%). All 22 (62.9%) of 35 patients who did not have scoliosis did not develop scoliosis with GH therapy. Since abnormal paraspinal muscle development was thought to induce scoliosis, we measured cross-sectional areas of paraspinal muscles by using one slice CT scan at the level of the umbilicus at the level of L4. Since there was a delay in the increase in total paravertebral muscle area and prolonged asymmetry in patients with progressive scoliosis, both were thought to be useful predictors of progressive scoliosis in PWS patients with GH therapy. © 2012 Wiley Periodicals, Inc.