We report a case of mucoepidermoid carcinoma of the thyroid gland. The simultaneous association of papillary and mucoepidermoid carcinoma in a Hashimoto's thyroiditis makes the present observation unusual. Surgery was limited due to local extension of the neoplasm. The patient consequently underwent external radiotherapy followed by radiometabolic therapy. The patient survived 11 months after diagnosis. As far as the histogenesis of the neoplasm is concerned, we believe that mucoepidermoid areas correspond to squamous and mucinous metaplasia of a preexisting papillary carcinoma. Transition areas between elements morphologically characteristic of both neoplasms were observed. Histochemical and immunohistochemical studies confirmed the diagnosis of a carcinoma with multiple aspects showing a focal positive reaction for thyroglobulin or keratin antisera. Therefore, this demonstrates different functional activities of the neoplastic cells. There is still debate about adjuvant therapies, the results of which appear for the moment very poor.