PURPOSE: To compare the usefulness of orbital ultrasound measurements of extraocular muscle thickness with that of CT measurements in patients with thyroid ophthalmopathy. MATERIALS AND METHODS: Fourteen patients with thyroid ophthalmopathy underwent orbital ultrasound and CT scanning, and 19 normal volunteers underwent ultrasound. For orbital ultrasound, a 5-10 MHz small part probe was applied to bilateral closed eyelids. The medial and lateral rectus muscles were measured on transverse scan, and the superior rectus complex and inferior rectus muscles on longitudinal scan. On orbital CT, the medial and lateral rectus muscles were measured on axial scan, and the superior rectus complex and inferior rectus muscle on coronal scan. The maximum thickness of the belly of each muscle was measured on both ultrasound and CT. The ultrasound detection rates for each measurable muscle were assessed in both the thyroid ophthalmopathy and normal control group, and the statistical significance of the thickness of extraocular muscles measured by ultrasound and CT was evalvated. RESULTS: In patients with thyroid ophthalmopathy and in the normal control group, all medial and lateral rec-tus muscles were successfully measured by ultrasound. The detection rate for the superior rectus complex was 60.7 % in the thyroid ophthalmopathy group and 63.1% in the normal control group, and for the inferior rectus muscle, 78.6% and 39.5% in the two respective groups. A comparison of measurements of the thickness of extraocular muscle using orbital ultrasound and CT showed no statistically significant difference between the medial rectus muscle and the superior rectus complex, but a significant difference between the lateral and inferior rectus muscle. CONCLUSION: In these patients, all medial rectus muscles were easy measurable by ultrasound, and the thickness thus determined was not different from that determined by CT. Orbital ultrasound is thus a useful method for diagnosis and follow-up in patients with thyroid ophthalmopathy in whom the medial rectus mus-cle is thickened.