The purpose of the study was to analyze the results after in situ posterolateral arthrodesis without reduction in children and adolescents with Meyerding grades III and IV spondylolisthesis and in patients with spondyloptosis who had an average follow-up of 12.8 years. The study population consisted of 21 patients who underwent an in situ posterolateral spinal fusion from L4 to S1 with autogenous iliac bone graft and were immobilized in a pantaloon cast for 4 months. All patients reported improvement after the operation and had no limitation in daily activities. Only four of 21 patients complained of occasional mild pain after physical activity, which resolved with rest and did not disturb their work. After surgery there were no motor deficits, incontinence of bowel or bladder, or sexual dysfunction. Roentgenographic findings showed progression of the slip in five patients and increase of the slip angle and the displacement index in two patients. These changes happened during the first year after the operation. Progression of the slip was not associated with symptoms. There was no pseudarthrosis. In situ posterolateral arthrodesis with a large amount of autogenous bone graft followed by immobilization in a pantaloon cast yields a solid arthrodesis and provides satisfactory results.