Introduction: Progressive and non-juvenile avascular osteonecrosis (AVN) is a rare condition in children. During the last decade, some data indicate that regional deep hyperthermia therapy (RHT) combined with either chemo- and / or radiotherapy in malignancies is associated with AVN in young patients. In this study, we present our data on AVN following RHT in children with intra-pelvic malignancies. Material and Methods: Localization, extent of AVN, and associated joint effusions were evaluated via MRI and X-ray findings in 37 patients treated with RHT and chemotherapy ± additional radiotherapy for intra-pelvic malignancies in our study. AVN was classified in accordance to the Association Research Circulation Osseus (ARCO). In addition, the recurrence of sarcoma after RHT, the number of total joint replacements, and level of activity including sport activities were recorded in all patients. The mean follow-up was 6.2 years (SD: 4.1, range: 1-12 years). Results: Eight out of 37 pediatric patients treated with RHT and chemotherapy ± additional radiotherapy showed AVN of the femoral head within our follow-up period. Five out of the eight children developed bone marrow edema within 6 months after RHT procedure and three additional patients within the first year. All patients except one showed a rapid progression of AVN from ARCO stage 0 to the postcollapse- stages III and IV in our study. Seven out of eight AVN patients survived without evidence of further malignancy. Although advanced stages of AVN were observed in our patient group, they were able to still maintain a high quality of life. No patients in our group have undergone total hip replacement thus far. Conclusion: Based on our findings, we hypothesize a high risk of AVN in young children who receive RHT for pelvic sarcoma. However, further clinical investigation needs to be done to prove our hypothesis.