Fifty-three adult idiopathic scoliosis patients whose fusions ended caudally between L2 and S1 were reviewed relative to criteria for selection of the lowest fused vertebra and their outcome. The Harrington stable zone, central sacral line, and the presence of rotary subluxation, arthritis, spondylolisthesis, vertebral body and disc space wedging were studied. Preoperative and postoperative pain, quality of life, and decisions for surgery were surveyed with a 94% response. Results showed the older the patient, the lower the fusion. The Harrington stable zone was useful, whereas the central sacral line frequently indicated longer fusions by as much as three segments. The presence of pathoanatomic features also dictated lower fusions. Patients in whom the lowest level of fusion was consistent with selection criteria had reduced frequency and intensity of low-back pain.