Although a variety of eye muscle antigens are recognized by autoantibodies in the serum from patients with thyroid associated ophthalmopathy (TAO) 64 kDa membrane proteins, which are also expressed in the thyroid, are most closely linked to the development of ophthalmopathy in patients with autoimmune thyroid disease. A cloned 64 kDa protein called 1D, which shares homology with tropomodulin, appears to be closely related to a 64 kDa eye muscle protein identified in immunoblotting and both may be members of a family of cytostructural proteins. The relationship between 64 kDa proteins in eye muscle and thyroid, and its equivalent in somatic skeletal muscle, will only be understood when the various proteins are cloned from expression libraries, sequenced and their consensual and unique domains identified. Since these 64 kDa antigens are expressed in both thyroid and eye muscle, a possible mechanism for the association of ophthalmopathy with autoimmune thyroid disease is immunological cross-reactivity by autoantibodies and sensitized T lymphocytes. Autoantibodies reactive with 64 kDa eye muscle proteins are associated with ophthalmopathy in patients with autoimmune thyroid disorders and predictive of the development of ophthalmopathy in patients with Graves' hyperthyroidism.