Introduction. The potential structural effect of the sacral tilt on the development of disc wedging adjacent to lowest instrumented vertebra (LEV), and consequent postoperative coronal trunk decompensation in the surgical treatment of idiopathic scoliosis are well known. The aim of this study is firstly to establish the possible role of sacral tilt, limb length inequality, and associated pelvic girdle anomalies in the cause of idiopathic scolisosis, and secondly to determine the effect of sacral tilt on adjacent disc wedging below LEV in some idiopathic curves such as thoracolumbar/lumbar curves, and double major curves postoperatively. Methods. Between 2006 and 2008, 159 patients with idiopathic scoliosis from outpatient clinics were included in analyses. In all patients, standing posteroanterior and lateral spinal column and Ferguson radiographs were routinely taken. If direct radiographs suggested iliac bone asymmetry, hemi-pelvis volume measurements were done with CT scan. Curve type, sacral tilt, L5 tilt, iliac asymmetry, and limb-length differences were measured. As a second part of study, 87 surgically treated patients with double major curves were examined retrospectively in terms of adjacent subjacent disc wedging below LEV and sacral tilt. Results. In the first part of the study, the mean age of patients was 14·21 years (range 2–25) and all but 14 were female. The curve types were lumbar in 31 patients, thoracolumbar in 42, double major in 55, thoracic in 27, and triple in four. Sacral tilt towards the convex side of TL/L curve was present in 117 (74%) patients with an average of 8° measured from Ferguson graphs. L5 tilt towards the convex side of TL/L curve, and limb length inequality of less than 5 mm on the convex side of TL/L curve were more prevalent in patients with sacral tilt (57%) than in those without sacral tilt (p=0·021). On hemi-pelvis volume measurement, patients with no sacral tilt (n=42) had no difference in both sides whereas patients with sacral tilt (n=117) showed significant difference, with the convex side being smaller. We noted a correlation between L5 tilt and sacral tilt (p=0·048) and between sacral tilt and hemi-pelvis volume (p=0·024). There was no correlation between sacral tilt and pelvic asymmetry, age, sex, curve type, or amount of limb length inequality. In the second part of the study, disc wedging adjacent to LEV was present in 75 of 87 patients and sacral tilt was present in 72 of 75 (96%) (p=0·008). Six patients had limb length inequality of more than 1 cm, showing the postoperative trunk decompensation, which was corrected by elevating the shorter leg by shoe-lift. Conclusions. This study revealed significant association between sacral tilt and hemi-pelvis hypoplasia independent from age. This association may have a primary role in the cause of this disease. Although LEV is paralel to pelvis, intervertebral disc adjacent to LEV is often wedged and there is lumbosacral hemicurve distally below the instrumentation in patients treated by either anterior or posterior surgery. We believe that lumbosacral hemicurve due to sacral tilt behaves like hemivertebra, which is impossible to detect with preoperative standing anteroposterior radiographs. Ferguson view is necessary to detect the sacral tilt. The postoperative trunk decompensation is greater with presence of sacral tilt and limb length inequality