Thirteen patients with giant cell tumors of bone have been treated by radiation therapy because surgery was not feasible or unacceptably disfiguring. Seven patients were treated for primary giant cell tumors of the bone, four for recurrent disease, and three for metastasis (one presented with both distant metastasis and local recurrence after primary surgery). The follow-up time ranged from 18 months to 13 years, with a mean of 6.5 years. All patients except one are alive. Local control was achieved in 11 patients (85%). One patient whose tumor was located in the sacrum had no gross response and at 5 months was subjected to a partial sacrectomy. A second patient had local regrowth 1 year after treatment; salvage surgery was successful. There have been no long-term complications of radiation therapy. This study confirms that for patients with giant cell tumor of bone, radiation therapy offers an effective alternative to complex or difficult surgery and constitutes a good treatment method to medically inoperable patients.