For giant cell tumors of bone, does radiotherapy provide a safe and effective treatment? This retrospective review includes 24 patients with 26 histologically diagnosed tumors treated with megavoltage radiotherapy between March 1972 and July 1996. Of the 10 recurrent tumors, five had an intralesional resection, two had a biopsy, and three had no biopsy before radiotherapy. Of the 16 previously untreated tumors, one was irradiated after a marginal resection, five after an intracapsular resection, and 10 after biopsy alone. The total doses ranged from 35 to 55 Gy (median, 43 Gy) in fractions of 1.67 to 2.33 Gy per day. Twenty of 26 tumors (77%) were controlled locally. All of the local recurrences occurred within the irradiated field. Five of six patients with local recurrence were treated successfully with additional surgery. Salvage surgery after local recurrence required amputation of an extremity in three patients and a total knee replacement in one patient. The ultimate local control rate was 96% with one patient alive with progressive disease. Lung metastases in one patient were treated successfully with surgery, chemotherapy, and radiotherapy. In one patient a radiation-induced sarcoma developed 22 years after treatment. The authors conclude that radiation therapy is a safe and effective treatment option for benign giant cell tumors of bone. A total dose greater than 40 Gy is the only variable found to significantly influence local control.