Thyroid-associated ophthalmopathy is thought to be an autoimmune disease affecting the orbit. The precise pathogenetic mechanisms are not known, but extraocular muscle and/or orbital fibroblasts are the likely targets of the autoimmune attack. Sera from 41 normal controls, 79 patients with thyroid-associated ophthalmopathy and 72 patients with other autoimmune diseases were examined for antibodies to cultured orbital fibroblasts and extraocular muscle by enzyme-linked immunosorbent assay. Orbital fibroblast antibody levels varied widely in all subject groups studied, and failed to distinguish patients with thyroid-associated ophthalmopathy from patients with other autoimmune diseases or controls. Eye-muscle binding antibody levels were higher amongst patients with ophthalmopathy compared to normal controls and patients with Graves' hyperthyroidism without clinical evidence of ophthalmopathy. Furthermore, eye-muscle binding antibody levels were found to be particularly high in patients with ophthalmopathy and concurrent dermopathy, and in patients with ophthalmic (euthyroid) Graves' disease.