Summary The pathology of the orbital changes in Graves' ophthalmopathy (GO) has been discussed in detail. The target tissue is the eye muscle and the damage probably results from autoimmune processes. Cell-mediated immune responses have been demonstrated and an antibody to eye muscle is detectable in 70% of patients. There is no direct evidence for an effect of thyroid hormones or TSH on orbital tissues in GO. The relation between GO and autoimmune thyroid disease is discussed. Present evidence suggests that in GO autoimmune responses are directed to orbital tissue antigens and do not cross-react with thyroid antigens. Clinical studies suggest that all patients with hyperthyroidism have some abnormality of eye muscle, whereas not all patients with ophthalmopathy have evidence of thyroid disease. It is not possible at the present time to be certain whether GO is an integral part of Graves' disease or a separate entity.