We have carried out a prospective study to investigate whether orbital radiotherapy combined with high dose systemic glucocorticoids is more effective than orbital radiotherapy alone for Graves' ophthalmopathy. Thirty consecutive patients with relevant and active Graves' ophthalmopathy were randomly assigned to treatment either with orbital radiotherapy combined with systemic glucocorticoids (Group 1, n = 15) or with orbital radiotherapy alone (Group 2, n = 15). The final evaluation was made 6-9 months after beginning treatment. Two patients in each group were lost to follow-up. Ocular involvement and response to treatment were evaluated by the ophthalmopathy index and by clinical assessment. Mean ophthalmopathy index values were 5.85 in Group 1 and 5.46 in Group 2 (p = NS) before treatment, and 2.46 in Group 1 and 3.61 in Group 2 after treatment (p = 0.0001 and p = 0.003 vs initial value, respectively). The mean ophthalmopathy index decrease in Group 1 (-3.39) was significantly greater (p = 0.043) than that in Group 2 (-1.85). Favorable responses on clinical ground occurred in 9 patients (69%) in Group 1 and in 5 patients (38%) in Group 2. The difference was particularly evident on soft tissue changes and extraocular muscle involvement. Severe eye muscle restriction was substantially unaffected by either treatment. In conclusion, the association of orbital irradiation and high dose systemic glucocorticoids in the treatment of severe Graves' ophthalmopathy provides more favorable responses than orbital radiotherapy alone.