Undifferentiated (anaplastic) thyroid carcinoma is a highly aggressive neoplasm which may simulate microscopically a variety of soft tissue sarcomas. The three major subtypes are spindle cell, giant cell, and squamoid. Electron microscopic examination and immunocytochemical stain for cytokeratin are the most useful special techniques to document the epithelial nature of the tumor. Undifferentiated small-cell carcinoma is a term to be discarded. Nearly all tumors so designated in the past are malignant lymphomas, small-cell variants of medullary carcinomas, or poorly differentiated (insular) carcinomas. Poorly differentiated (insular) carcinoma is a thyroid malignant tumor characterized by the formation of solid nests and small follicles, a monotonous growth of small cells of follicular derivation, mitotic activity, necrosis, and peritheliomatous formation. Immunocytochemically, it is negative for calcitonin and positive for thyroglobulin. It is a clinically aggressive neoplasm, which metastasizes to both regional lymph nodes and distant organs. It is probably analogous to the tumor described by Langhans in 1907 as "wuchernde Struma."