Three patients with lumbosacral agenesis underwent surgery to lock the lumbopelvic instability. All three patients had an unstable sitting position and a kyphotic bearing that impaired intestinal transit or hampered further colostomy or ureterostomy. Luque instrumentation with iliac fixation performed according to Galveston was used in one patient. Cotrel-Dubousset instrumentation was used for the two other patients. Autografts plus allografts provided sufficient bone for fusion without requiring lower leg amputations. Increased hip flexion was obtained after pelvic stabilization, but knee flexion contracture remained the same. All patients showed improved intestinal transit or decreased urinary infections, and two patients attained a stable sitting position without aid.