Correction surgery for rigid adult spinal deformity usually involves a complex 360° osteotomy, multiple intraoperative position changes, and staged surgery. Moreover, there is a lack of consensus regarding the surgical strategy for this pathology. We report the technical advantages of a simultaneous anterior and posterior release only in the lateral decubitus position to reduce surgical invasiveness in two case reports. A 76-year-old woman and an 80-year-old woman presented with significant spinal imbalance and segmental fusion in the anterior and posterior columns around the apex of the lumbar spinal curvature. We conducted this procedure for these patients at the first stage of spinal corrective surgery to achieve 360° osteotomy. A long posterior fusion surgery was performed after 1 week. The mean values of the central sacral vertical line, pelvic incidence minus lumbar lordosis, sagittal vertical axis, and pelvic tilt improved substantially postoperatively: central sacral vertical line, from 51.0 to 7.5 mm; pelvic incidence minus lumbar lordosis, from 27.5° to 0.5°, sagittal vertical axis, from 107.6 to 14 mm; pelvic tilt, from 34.0° to 13.0°. The mean surgical time and blood loss in the first- and second-stage operations were 242.1 minutes and 702 mL and 315.5 minutes and 549 mL, respectively, and no perioperative complications occurred. Simultaneous 360° segmental release in the lateral decubitus position without repositioning can make it possible to acquire satisfactory correction and reduce surgical invasiveness compared with the conventional procedure for adult spinal deformity.