We retrospectively reviewed the results, particularly with regard to complications, of lengthening of long bones in eight children (nine limb segments) who had a limb-length discrepancy secondary to an underlying bone disorder (Group 1). The mean age of these patients was twelve years (range, six to sixteen years), the mean preoperative limb-length discrepancy was 6.0 centimeters (range, 2.7 to 8.8 centimeters), and the mean lengthening of the nine limb segments was 6.2 centimeters (range, 2.7 to 9.0 centimeters). Only two extremities were equalized.We compared the results in Group 1 with those of limb-lengthening in seven children (nine limb segments) who had a discrepancy secondary to post-traumatic growth arrest (Group 2) and seven children (seven limb segments) who had a discrepancy secondary to growth arrest following an infection in the bone (Group 3). All of the procedures were performed at our institution during the same time-period by the same surgeons.There were forty-one complications (twenty-five minor and sixteen major), with a mean of five complications per limb segment, in Group 1; twenty-six complications (twenty minor and six major), with a mean of three complications per limb segment, in Group 2; and twenty-two complications (fourteen minor and eight major), with a mean of three complications per limb segment, in Group 3.The results in Group 1 suggest that the Ilizarov technique for lengthening, although effective in restoring the length of the extremity, is associated with a higher rate of complications in patients who have a discrepancy due to an underlying bone disorder than in those who have a discrepancy due to growth arrest. Therefore, caution should be exercised before a lengthening procedure is recommended for a patient who has an underlying bone disorder.