The influence of surgery on the relationship between the lumbar spine and the pelvis in adolescent idiopathic scoliosis (AIS) remains mostly unknown. The sagittal spinopelvic balance of 40 patients with AIS undergoing posterior spinal instrumentation and fusion was studied. After surgery, the total lumbar lordosis (LL) and the LL below fusion remained correlated to the sagittal pelvic geometry. LL within fusion, which is set by the instrumentation, was also related to the LL below fusion. This preliminary study shows that pelvic morphology probably has an important impact on the postoperative sagittal lumbar alignment. Evaluation of the sagittal pelvic geometry could therefore be useful in the preoperative planning of surgery for AIS. Further studies with more patients are still needed in order to confirm this hypothesis and to evaluate if the distal fusion level influences the spino-pelvic balance.