We examined 13 patients with euthyroid Graves' disease suspected ophthalmologically, by comparing them with 20 patients with untreated Graves' disease and by following them up for 5 to 10 years. They had Graves' ophthalmopathy (NOSPECS class II-IV) without other ocular diseases, normal levels of serum thyroid hormones, and no previous history of Graves' disease. Proptosis in euthyroid Graves' disease was not significantly different from that in untreated Graves' disease. In 3 patients with euthyroid Graves' disease, TSH was suppressed. There was either no TSH response to TRH or it was low in 7 of 12 patients examined. The result of a T3-suppression test was abnormal in 8 of 11 patients examined. Titers of serum TGHA, MCHA, TSH-binding inhibitory immunoglobulin (TBII), and thyroid stimulating antibody (TSAb) were significantly lower in patients with euthyroid Graves' disease compared than in patients with untreated Graves' disease. TSAb, however, was positive in 12 of 13 (92%) patients. In spite of positive TSAb, 9 of 13 patients with euthyroid Graves' disease had normal radioactive iodine uptake (RAIU). During the observation period, various abnormalities in thyroid function developed: persistent hyperthyroidism in 5 patients (38%), transient thyrotoxicosis in 2 (15%) and transient hypothyroidism in 1 (8%). We conclude that euthyroid Graves' disease is a subtype of Graves' disease that minimally develops thyrotoxicosis in spite of the existence of TSAb due to some mechanism inhibiting thyroid growth or stimulation, and that the measurement of TSAb provides a useful marker for the diagnosis of this disease.