Twenty-five cases of a distinctive type of thyroid carcinoma are presented. The key microscopic features are: formation of solid clusters ("insulae") of tumor cells containing a variable number of small follicles; small size and uniformity of the tumor cells; variable, but consistently present mitotic activity; capsular and blood vessel invasion; and frequent necrotic foci, sometimes leading to the formation of "peritheliomatous" structures. Metastases to regional lymph nodes, lung, and bones are common, and these often lead to the patients' death. We view this tumor as a poorly differentiated carcinoma, situated morphologically and biologically in an intermediate position between the well-differentiated (papillary and follicular) and the totally undifferentiated thyroid tumors. We believe that it is closely related, if not identical, to the tumor type described by Langhans in 1907 as "wuchernde Struma."