Twenty-two adolescent patients with severe (more than 50%) slip were surgically treated. Eleven were reduced with Magerl/Dick transpedicular screw devices and fused posteriorly from L4 to S1, and 2 weeks later anteriorly L5-S1; the other 11 were fused in situ L4-S1 (6 patients) or L5-S1 (5 patients) using a circumferential (6 patients), anterior (4 patients) or posterolateral (1 patient) technique without instrumentation. The two groups were comparable as to age at operation, age at follow-up, follow-up time, and preoperative radiologic measurement of the slip, lumbosacral kyphosis, and clinical findings. The mean follow-up times were 56.5 and 59.8 months, respectively. In the reduction group an improvement in the slip of 36.1 percentage points was achieved as compared with 7.7 percentage points in the in situ-fusion group. The sagittal rotation angle improved by 11 in the reduction group and worsened by 2.8 in the in situ-fusion group. There were no differences between the groups in the functional tests or clinical findings concerning pain. Subjective assessment was good in both groups at follow-up; that is, the pain had disappeared. Mean operation time and intraoperative blood loss were significantly higher in the reduction group. Reduction procedures were also associated with a higher number of complications and reoperations. No neurologic complications, however, occurred in the reduction group. Based on this study, in situ fusions are to be preferred in adolescents with severe spondylolisthesis.