Seventy cases of anaplastic thyroid carcinomas studied at the Universities of Florence (Italy) and Minnesota are presented. Three morphologic patterns were seen: spindle, giant cell, and squamoid, sometimes in combination. Ultrastructurally, evidence of epithelial differentiation was seen in most but not all cases studied. Immunohistochemically, a stain for cytokeratin using a monoclonal antibody was found the most useful adjunct to diagnosis. Unexpected positivity for carcinoembryonic antigen (CEA) was found in several squamoid tumors. The alleged frequent positivity of this tumor type for thyroglobulin and calcitonin was not confirmed. A third of the tumors were associated with a better differentiated component, of which, presumably, they represented a dedifferentiation. The extremely aggressive behavior of anaplastic thyroid carcinomas was confirmed amply in this series: all of the patients in whom follow-up information was available died of their tumor. Small cell tumors should not be included into the anaplastic category, since they invariably belong to other groups, i.e., malignant lymphoma, medullary carcinoma, and poorly differentiated ("insular") carcinoma.