OBJECTIVE Spinal osteotomy in pediatric patients is challenging due to various factors. For correction of severe rigid kyphoscoliosis in children, numerous techniques with anterior or posterior or combined approaches, as well as multilevel osteotomies, have been described. These techniques are associated with prolonged operative times and large amounts of blood loss. The purpose of this study was to evaluate the clinical and radiologically confirmed efficacy of a modification of the apical spinal osteotomy (ASO) technique-posterior-only single-level asymmetric closing osteotomy-in pediatric patients with severe rigid kyphoscoliosis. METHODS The authors performed a retrospective study of a case series involving pediatric patients with severe spinal deformity operated on by a single surgeon at a single institution over a period of approximately 5 years. The inclusion criteria were age < 14 years, rigid thoracic/thoracolumbar/lumbar kyphosis (> 70°) with or without neurological deficit and with or without scoliosis, and a minimum of 2 years of follow-up. Patients with cervical or lumbosacral kyphoscoliosis were excluded from the study. Demographic and clinical parameters, including age, sex, etiology of kyphoscoliosis, neurological examination status (Frankel grade), and visual analog scale (VAS) and Oswestry Disability Index (ODI) scores, were noted. Operative parameters (level of osteotomy, number of levels fused, duration of surgery, blood loss, and complications) were also recorded. Radiological assessment was done for preoperative and postoperative kyphosis and scoliosis as well as the final Cobb angle. Similarly, sagittal vertical axis (SVA) correction was calculated. Fusion was assessed in all patients at the final follow-up evaluation. RESULTS A total of 26 pediatric patients (18 male and 8 female) with a mean age of 9 years met the inclusion criteria and had data available for analysis, and all of these patients had severe scoliosis as well as kyphosis. Comparison of preoperative and postoperative values showed a significant improvement (p < 0.05) in radiological, clinical, and functional parameters (Cobb angle for scoliosis and kyphosis, SVA, VAS, and ODI). With respect to kyphosis, the mean preoperative Cobb angle was 96.54°, the mean postoperative angle was 30.77°, and the mean angle at final follow-up was 34.81° (average loss of correction of 4.23°), for a final average correction of 64.15%. With respect to scoliosis, the mean preoperative angle was 52.54°, the mean postoperative angle was 15.77°, and the mean angle at final follow-up was 19.42° (average loss of correction of 3.66°), for a final average correction of 60.95%. The preoperative SVA averaged 7.6 cm; the mean SVA improved to 3.94 cm at the end of 2 years. Bony fusion was achieved in all patients. The mean number of levels fused was 5.69. The mean operative time was 243.46 minutes, with an average intraoperative blood loss of 336.92 ml. Nonneurological complications occurred in 15.39% of patients (2 dural tears, 1 superficial infection, 1 implant failure). At the 2-year follow-up, 25 of the 26 patients had maintained or improved their neurological status. One patient developed paraplegia immediately after the operation and recovered only partially. CONCLUSIONS Analysis of data from this series of 26 cases indicates that this posterior-approach single-level technique is effective for the correction of severe rigid kyphoscoliosis in pediatric patients, providing good clinical and radiological results in most cases.