Sagittal spinopelvic relations have been reported in adolescent idiopathic scoliosis (AIS), but there is little information on their effect following surgery. The objective of this study is to evaluate the relation between the pelvic and lumbar spine geometries following posterior spinal instrumentation and fusion (PSIF). Sixty patients with AIS undergoing PSIF were studied retrospectively. Thoracic kyphosis (TK), lumbar lordosis (LL), LL within and below fusion, pelvic incidence (PI), sacral slope (SS) and pelvic tilt (PT) were measured on preoperative and postoperative standing lateral radiographs. Significant postoperative correlations were found between PI and LL (r = 0.67), SS and LL (r = 0.90), PI and LL below fusion (r = 0.40), SS and LL below fusion (r = 0.48). Pelvic parameters did not influence LL within fusion. A strong correlation was found between LL below and within fusion (r = -0.76). The close interdependence between lumbar lordosis and pelvic geometry preoperatively is maintained postoperatively following PSIF. In the planning of surgery for AIS, it may be helpful to evaluate the sagittal pelvic morphology (PI) in addition to the spinal curves. Preoperative evaluation of the pelvic morphology could be used to optimize intraoperative positioning of the patient and to determine the optimal amount of LL that needs to be restored or preserved by the instrumentation, so that LL remains congruent with the pelvic morphology.