We attempted to determine if euthyroid Graves's ophthalmopathy is a single entity or a heterogeneous group of disorders. Activity of long acting thyroid stimulator protector occurred in 31 of 33 patients with Graves's hyperthyroidism but in only nine of 17 with euthyroid Graves's ophthalmopathy. Of the euthyroid patients, six had protector activity and thyroid non-suppressibility; firm goiters and high titers of thyroid antibodies were the rule in this group. We believe that these patients have three autoimmune diseases: Hashimoto's thyroiditis, Graves's thyroid disease and Graves's ophthalmopathy. Five euthyroid patients had no detectable protector activity or thyroid antibodies and had normal thyroid suppressibility; the thyroid was generally normal in size and consistence. These patients are interpreted as having "isolated" Graves's ophthalmopathy without any autoimmune thyroid disease. The remaining six patients showed dissociation between protector activity and thyroid non-suppressibility and cannot be classified as yet. Euthyroidism in Graves's ophthalmopathy may have more than one cause.