12 of 17, a significant frequency (71%), of untreated Graves' disease patients with no clinical ophthalmopathy showed extraocular muscle (EOM) enlargement by Magnetic Resonance Imaging (MRI). Enlargement was bilateral in 41% and unilateral in 29% in these patients. Apparent enlargements of EOM were also detected, by MRI, in all of 11 Graves' disease patients with clinical ophthalmopathy, bilateral in 73% and unilateral in 27% of patients in this group. Both group showed the inferior rectus muscle as the most frequently involved (56% and 77% respectively). In 16 patients without autoimmune thyroid disorders or ophthalmopathy who served as normal controls, only 2 of these patients (12%) demonstrated mild EOM enlargement. The severity and patterns of EOM enlargement revealed no correlation with abnormalities in serum thyroid function tests or serum thyroidal autoantibodies. In conclusion, a high frequency of Graves' disease patients without clinical eye signs or symptoms harbor EOM abnormalities, as demonstrated by MRI. This suggests that present clinical examination methods are insufficient to diagnose varying degrees of ophthalmopathy in patients with autoimmune thyroid disorders who do not initially present with clinical ophthalmopathy.