Most medullary thyroid carcinomas show suspicious malignant features such as hypoechogenicity, a spiculated margin and/or intranodular calcifications, which are well known features of papillary carcinoma. We report here on a case of medullary carcinoma that was seen as a predominantly cystic thyroid mass on ultrasonography. This type of case is not common in the literature and we discuss the way to diagnose a medullary thyroid carcinoma.