Background. The best treatment (steroids, irradiation, or both) for moderately severe Graves' orbitopathy, a self-limiting disease is not known. We tested the efficacy of external beam irradiation compared with sham-irradiation. Methods. In a double-blind randomised clinical trial, 30 patients with moderately severe Graves' orbitopathy had radiotherapy (20 Gy in ten fractions), and 30 were assigned sham-irradiation (ten fractions of 0 Gy). Treatment outcome was measured qualitatively by changes in major and minor criteria and quantitatively in several ophthalmic and other variables, such as eyelid aperture, proptosis, eye movements, subjective eye score, and clinical-activity score at 24 weeks. Findings. The qualitative treatment outcome was successful in 18 of 30 (60%) irradiated patients versus nine of 29 (31%) sham-irradiated patients at week 24 (relative risk [RR] = 1.9 [95% CI 1.0-3.6], p = 0.04). This difference was caused by improvements in diplopia grade, but not by reduction of proptosis, nor of eyelid swelling. Quantitatively, elevation improved significantly in the radiotherapy group, whereas all other variables remained unchanged. The field of binocular single vision was enlarged in 11 of 17 patients after irradiation compared with two of 15 after sham-irradiation. Nevertheless, only 25% of the irradiated patients were spared from additional strabismus surgery. Interpretation. In these patients with moderately severe Graves' orbitopathy, radiotherapy should be used only to treat motility impairment.