The value of an osteotomy of the fusion mass in attaining realignment of the spine when a lateral shift of the trunk over the pelvis has developed in the frontal plane was studied in fifty-five patients. All had had a previously fused but progressive spinal deformity. The indications for osteotomy were progression of the curve, imbalance of the trunk, pain, deteriorating cardiopulmonary function, or pseudarthrosis, or a combination of these conditions. Twenty-one patients had idiopathic scoliosis; fourteen, congenital scoliosis; ten, paralytic scoliosis; four, myelodysplastic scoliosis; and six had scoliosis secondary to miscellaneous conditions. A total of 154 osteotomies of the fusion mass, or 2.8 osteotomies per patient, were performed. Nine patients underwent an anterior spinal osteotomy or wedge resection as well. Most had a two-stage procedure which consisted of a posterior osteotomy, halo-femoral traction, and a posterior re-fusion with Harrington instrumentation two weeks later. At an average follow-up of thirty-two months, it was found that osteotomy of the fusion mass enabled the surgeon to realign the trunk over the pelvis and to correct pelvis obliquity, but correction of the angular deformity was only secondary. Although the rate of complications was high (51 per cent, including one postoperative death), osteotomy of the fusion mass and re-fusion may be indicated in a patient with a previously fused but progressively unbalanced, painful deformity of the spine.