A case is presented of an anterior mediastinal mass arising in a 64-yearold woman that showed on histologic, immunopathologic, and ultrastructural examination features of a poorly differentiated (insular) thyroid carcinoma. The tumor in this patient most likely arose from ectopically displaced thyroid tissue on the basis of a developmental defect. The clinicopathologic features and differential diagnosis of the lesion in the setting of its mediastinal location are discussed. Ectopic poorly differentiated (insular) carcinoma of the thyroid should be added to the list of mediastinal tumors showing a solid or insular growth pattern.