PURPPOSE: To demonstrate sonographic features of Hashimoto' s thyroiditis according to the thyroid function. MATERIALS AND METHODS: We reviewed 54 thyroid ultrasonographic examinations of untreated Hashimoto' s thyroiditis. We reviewed thyroid ultarasonographic examinations and focused on the presence of ill defined low echoic lesion and glandular enlargement. We performed another thyroid ultrasonographic examination of 14 healthy volunteers, in order to obtain normal size of thyroid gland. Comparision was made between these morphologic charecteristics and functional stage of the disease. RESULTS: The mean diameter of thyroid gland was 2.16 +/-0.43cm in patients with Hashimoto' s thyroiditis, and 1.41 +/-0.42cm in normal control group of the thyroid gland. There was no statistically significant relationship between thyroid function and size. There was morphologic abnormalities in 46 patients (85%). Among them, 7 patients revealed diffuse low echogenity in the entire thyoid gland, 32 patients showed peripherally located, ill defined focal hypoechoic lesion, and 7 patients showed solitary or multiple, well defined nodular lesions. Decreased echogenity of the thyroid gland was related to hypothyroid status. CONCLUSION: Hashimoto's thyroiditis has specific morphologuc charecteristics in ultrasonographic features, which are well correlated with thyroid function.