OBJECTIVE To correlate exophthalmos with the volume of extraocular muscle and orbital fatty tissue in thyroid-associated ophthalmopathy (TAO), using MRI that enables the orbital soft tissues to be well defined. METHODS Thirty-three orbits, 20 from 10 patients with TAO and 13 from 13 controls, were employed. T1-weighted orbital MR slices 2 or 3 mm thick were obtained in axial, coronal and sagittal planes. Tracing the outlines of each structure, we measured the total sectional areas. Volumes of the extraocular muscle, of the fatty tissue and of the bony orbital cavity were calculated by multiplying the slice thickness. Exophthalmos was also measured using axial MRI. RESULTS In TAO the volume increment of orbital fatty tissue (6.19 cm(3)) was much greater than that of extraocular muscle (1.16 cm(3)). Increase of exophthalmos by 1 mm needed a total orbital volume increment of 0.92 cm(3). The total orbital fatty tissue volume (correlation coefficient 0.70, P=0.06%) and the anterior orbital fatty tissue volume (0.64, P=0.23%) were more closely correlated with the degree of exophthalmos than was extraocular muscle volume (0.58, P=0.8%). Moreover, the volume increment of extraocular muscle and orbital fatty tissue was not always proportional. CONCLUSIONS The results show that the orbital fatty tissue involvement is closely related to the degree of exophthalmos. For studying exophthalmos in TAO, the volumetric change, not only in ocular muscles, but also in orbital fatty tissue, should be taken into consideration.