With the cooperation of the Japanese Musculoskeletal Oncology Group (JMOG), we conducted a questionnaire survey to assess the status of extendable prosthesis use after resection of malignant bone tumors in children. The subjects were 28 patients (mean age, 10.1 years). Osteosarcoma was the most frequent tumor, and all patients had undergone preoperative chemotherapy and wide resection. The prosthesis was the Growing Kotz-type in 26 patients. The mean predicted leg length discrepancy was 67.7mm, mean stem diameter 10mm, mean number of elongations 2.1, and mean total elongation 35.4mm. Infection was observed in one patient with a mean functional assessment of 75%. In Japan, extendable prostheses, as represented by the Growing Kotz-type, provide a useful limb reconstruction method, with a minimum of major complications and with good function of the affected limb. However, a long-term follow-up survey is necessary.