Correction by Harrington instrumentation of six idiopathic scoliotic spines was studied through computer simulation. As the limit to lateral correction is approached, large force increases are needed to increase correction, beyond those likely to be achieved by improvements in hook fixation, for example. Significant derotation should not be expected in a Harrington procedure, and is likely to be achieved within a short time only by reducing intervertebral motion segment stiffnesses. Correction of the lateral curve and correction of the rotation were not dependent on one another.