Using B-mode ultrasonography, an attempt was made to measure the volume of extraocular muscles and retrobulbar fat in 31 patients (62 orbits) with Graves'disease. None of the patients had exophthalmometric measurements greater than 21mm or had eye symptoms. The mean value of muscle volume of Graves'patients was significantly larger than that of normal controls (6.48±2.70cm3 and 3.25±1.30cm3, respectively, p<0.001). All of the patients had extraocular muscle swelling, although 2 of them had no extraocular muscle change for their unilateral eye. The extraocular muscle volume increased as the degree of the proptosis increased. The fat volume tended to increase in parallel with the degree of the proptosis. In the Graves' group with obvious proptosis (Hertel reading: 19-21mm), the fat volume increased more significantly than in any other group. The ratio of extraocular muscle volume to retrobulbar fat volume was significantly higher in Graves' disease, but it did not increase as the degree of the proptosis increased. A significant correlation between proptosis and muscle volume plus fat volume was observed. No significant difference of the extraocular muscle volume was observed between the patients untreated and treated with antithyroid drugs. The data show a uniform enlargement of the extraocular muscles in Graves'disease and also suggest an involvement of increased retrobulbar fat volume in a group of obvious exophthalmos. The degree of the proptosis is in aclose proportion to the quantitative change of the orbital soft tissue.