Radiotherapy is often used in Graves' ophthalmopathy, but its efficacy has been doubted. We compared its efficacy with sham irradiation in mild ophthalmopathy. In a double-blind randomized trial, 44 patients received orbital irradiation, and 44 were sham-irradiated. The primary outcome was assessed using major and minor criteria. As secondary outcome, we used a disease-specific quality of life questionnaire (the GO-QoL) and compared cost-effectiveness and need for follow-up treatment. The primary outcome was successful in 23 of 44 (52%) irradiated patients vs. 12 of 44 (27%) sham-irradiated patients at 12 months after treatment (relative risk, 1.9; 95% confidence interval, 1.1-3.4; P = 0.02). Radiotherapy was effective in improving eye muscle motility and decreasing the severity of diplopia. However, quality of life improved similarly in both groups. In the radiotherapy group there was less need for follow-up treatment; 66% vs. 84% of the patients needed further treatment (P = 0.049). Retrobulbar irradiation did not prevent worsening of ophthalmopathy, which occurred in 14% of the irradiated and 16% of the sham-irradiated patients. Radiotherapy is an effective treatment in mild ophthalmopathy. However, the improvement upon irradiation may not be associated with an increase in quality of life or a reduction in treatment costs.