In cross section, extraocular muscles are more or less elliptical, with short and long diameters. We studied the ratio (R) of short to long diameter and investigated its use in quantitative assessment of the extraocular muscles in patients with Graves's disease. We measured the diameters on T1-weighted axial and coronal MRI and computed R for each extraocular muscle in 80 patients without and 40 with Graves's disease. We compared the measurements and R of the right and left orbits, and of men and women. The short diameter of all extraocular muscles apart from the superior oblique showed significant differences between men and women, and that of the inferior rectus varied significantly with age. R, however, was unrelated to sex or age. All patients with Graves's disease and an increased short diameter also had an increased R, but 6% of the muscles showed an increase in R, even though their short diameter was within the normal range.