Forty-one patients with idiopathic scoliosis having a primary right thoracic and a compensatory left lumbar curve underwent posterior spinal fusion of the primary curve only. Twenty-four patients had instrumentation with a Harrington rod or variant, and 17 patients underwent Cotrel-Dubousset instrumentation. Decompensation occurred postoperatively when the apex of the thoracic curve was located on or to the left of the center sacral line. There was no statistically significant change in the lumbosacral portion of the lumbar curve from the apex to the lumbosacral junction in both groups. Curve correction occurred cephalad to the apex of the lumbar curve, and not along the center sacral line. The decompensation rate was 4% for Harrington rod instrumentation and variants and 41% for Cotrel-Dubousset instrumentation. Cotrel-Dubousset instrumentation translated the apex of the thoracic curve 1.5 cm farther to the left than Harrington rod instrumentation and variants. When the apex of the lumbar curve is 2 cm or greater to the left of the center sacral line, the patient's spine will decompensate to the left, centered over the apex of the lumbar curve.