Eight adults who had back pain and sciatica that was caused by Grade-III or IV isthmic spondylolisthesis of the fifth lumbar vertebra on the sacrum were treated consecutively by in situ arthrodesis without decompression. The anterior displacement of the fifth lumbar vertebra averaged 82 per cent (range, 66 to 118 per cent). The length of follow-up averaged 5.5 years (range, two to fourteen years). All of the arthrodeses resulted in a solid fusion and excellent relief of both the back pain and the sciatica. All of the preoperative neurological deficits resolved, with the exception of a decreased or absent Achilles-tendon reflex in two patients. There were no complications, and all of the patients returned to their preoperative occupations.