The Authors describe a case, which came under their observation, of indifferentiated neoplasia of the thyroid associated with Hashimoto's disease; treated surgically by total thyroidectomy, at a distance of over four years it did not show signs of recurrence. Consideration is also given to pathogenetic and clinical problems, as well as those of surgical therapy, which such a combination between autoimmune thyroditis and a neoplastic process may raise.