The aim of the present study was to clarify the underlying molecules that might contribute to the highly aggressive behavior of anaplastic thyroid carcinoma. We selected 5 cases of anaplastic thyroid carcinoma that had a differentiated area to determine differences in the molecules of undifferentiated and differentiated cancer cells. We immunohistochemically examined the localization of nuclear antigen (Ki-67), proliferating cell nuclear antigen (PCNA), p53, apoptotic protease-activating factor-1 (Apaf-1), CD26, galectin-3, E-cadherin, and CD147. We found an increased Ki-67, PCNA, and p53 labeling indices; decreased levels of Apaf-1, CD26, galectin-3, and E-cadherin; and overexpression of CD147 in the undifferentiated area compared with the differentiated area. These findings indicate high proliferative properties, suppression of apoptosis, disruption of cell-cell interaction, and induction of matrix metalloproteinases in the undifferentiated areas. Thus the molecules examined might be useful for evaluating the aggressive nature of this tumor and the prognosis.