The surgical correction of leg length discrepancies during the growing years depends on an accurate prediction of future growth. In the case of leg lengthening procedures the aim is to lengthen the leg by that amount which will result in equal leg lengths at maturity, not by the amount of the current discrepancy which may be increasing. In the case of epiphyseodes is the principle is to slow down the growth of the long leg allowing the short leg to catch up, and accuracy therefore rests on the ability to predict not only the amount which the long leg will be slowed down, but also the ability of the short leg to catch up.