Over the past seven years, 96 PLIF operations were performed with the autologous bone graft method. In 44 cases, unicortical bone grafts were obtained from the posterior iliac crest area. In the remaining 52 cases, bicortical bone grafts were obtained from the anterior iliac crest. The failure rate in the patients treated by interbody fusion with the unicortical posterior iliac crest graft was 11.8%. The failure rate in patients treated by interbody fusion with bicortical bone grafts was only 1.9%. The patients were ambulatory three to four days after the operation. In cases of spondylolysis and spondylolisthesis, interbody fusion should be coupled with some manner of posterior internal fixation. The extraction of two pieces of bicortical bone from the anterior iliac crest, measuring 1.2 cm or 1.3 cm by 2.5 cm, may cause weakness of the pelvic bone. Therefore, the bicortical grafting in PLIF should be limited to severely disabled patients with complete spinal canal stenosis or very large myelographic defects.