Patients with severe Graves' ophthalmopathy have been treated by selective orbital irradiation using a well-collimated, high-energy, x-ray beam generated by a linear accelerator. Severity of involvement and response to therapy was judged by a numerical scoring procedure and by clinical assessment. Of 23 patients treated, 15 (65%) experienced an excellent or good response, which occurred soon after the start of radiotherapy. No patient was made worse by treatment. A favorable response to orbital irradiation was observed even in some patients who had previously responded poorly to systemic corticosteroid therapy. Severe visual acuity loss or elevated intraocular pressure were not considered to be contraindications to radiation treatment. Severe asymmetrical extra-ocular paresis was not relieved following treatment, even though other concomitant manifestations of ophthalmopathy improved. Anatomic, physiologic and immunologic concepts concerning the pathogenesis of the eye lesions constitute the rationale for the treatment employed. The fundamental pathologic process is thought to represent a cellular immune reaction in extra-ocular muscles.