This study reports the results of treatment of adolescents with King Types 2, 3, and 4 idiopathic curves using Cotrel-Dubousset instrumentation. Imbalance was seen in Types 2 and 4 curves when distraction direction hook patterns crossing the thoracolumbar junction were employed. Imbalance was not seen when a modified hook pattern employing compressing forces across the thoracolumbar junction was employed. No imbalance was observed in Type 3 curves using the basic right thoracic curve hook pattern. In Type 4 curves, a second modified hook pattern is required to obtain improved correction and balance. The mechanism of production of imbalance is explained by a three dimensional analysis of the deformity and of the forces generated by the Cotrel-Dubousset system.