OBJECTIVE To assess the outcomes of radiotherapy, in terms of local control and treatment complications, of advanced or difficult giant cell tumors of bone (GCTB) that could not be treated by surgery. METHODS Among 122 consecutive patients with confirmed GCTB from 1985 to 2007, 77 patients were treated by megavoltage radiotherapy because they were inappropriate candidates for surgery. We have performed analysis of all data in terms of progression-free survival (PFS) and treatment morbidity. Median follow-up time was 58 months. RESULTS In the irradiated group, maximal tumor size ranged from 5 to 18 cm (median, 8.5). Anatomic distribution was as follows: femur, 27 cases; tibia, 19; radial/ulnar bone, 12; sacrum, 9; pelvic bones, 5; other, 5. Twenty-one patients (27%) were referred for local recurrence after ≥1 other treatment procedures. The radiation doses ranged from 26 to 89 Gy (median, 56; administered 1.8-2.0 Gy/fraction with average total duration of treatment of 5-7 weeks); 8 patients (10%) received <50 Gy. All patients tolerated treatment well without acute or late complications. All patients except two are alive. Local control was achieved in 65 patients (84%; bone recalcification/restitution of joint functions), 12 patients showed signs of local progression, all within irradiated fields (9 were treated successfully with salvage surgery). Five- and 10-year local PFS were 83% and 73%, respectively. Three patients developed lungs metastases. Malignant transformation of GCTB occurred in two patients. CONCLUSIONS GCTB can be safely and effectively treated with megavoltage radiotherapy with local control rate >80% at 5 years. Our study confirms that radiotherapy of GCTB offers an alternative to difficult or complex surgery and may be an option of choice in the treatment of inoperable patients.