A 42-year-old woman suffered from pain in both legs, and fever. She was diagnosed with Erdheim-Chester disease (ECD), based upon bone biopsy. Initially, she received steroid therapy, which led to temporary improvement. However, bone pain in the right femur was so progressive that, as a trial course of radiation therapy, she was given a total dose of 18 Gy in ten fractions to the right distal femur. She showed a gradual response, and the local pain became controllable. Erdheim-Chester disease (ECD) is an uncommon, non-Langerhans form of histiocytosis, which is characterized by radiological and pathological findings, but its treatment is still controversial. We here present a representative case report of radiotherapy for ECD.