CD97 is a dimeric glycoprotein of M_r 75,000–85,000 and 28,000 belonging to a novel subfamily of seven-span transmembrane region leukocyte cell surface molecules. It is expressed abundantly in cells of hematopoietic origin. This is the first report demonstrating the expression of CD97 outside the hematopoetic system. CD97 was studied in normal human and neoplastic follicular epithelium of the thyroid and anaplastic (n = 3) and papillary (n = 1) thyroid carcinoma cell lines. In normal thyroid tissue (n = 11), no immunoreactivity of CD97 could be found, whereas in differentiated thyroid carcinomas (n = 10), CD97 expression was either lacking or low. Eleven of 12 undifferentiated anaplastic carcinomas revealed high CD97 presentation. CD97 was absent or only weakly present in patients with postoperative T₁ tumors but increased greatly with the progression to postoperative T₄ tumors. CD97 is clearly present in thyroid carcinoma cell lines but only at a very low level in normal human thyrocytes. Quantitation of CD97 cell surface expression levels revealed that C 643 and SW 1736 cells showed a two to four times higher specific antibody-binding capacity than did 8505 C and HTh 74 cells and a nearly 20 times higher specific antibody-binding capacity than normal thyrocytes. Phorbol 12-myristate 13-acetate treatment progressively caused a decrease of CD97 antigen expression in all cell lines to about 30% of their initial levels after 48 h. Immunohistochemical staining of SW 1736 cells revealed that CD97 is located in most of the cell compartments and suggested a CD97 internalization process after phorbol 12-myristate 13-acetate treatment. Semiquantitative reverse transcription-PCR showed a correlation of CD97 mRNA and cell surface CD97 expression level in the cell lines. SW 1736, HTh 74, and 8505 C cells apparently expressed CD97 with alternative glycosylation compared to peripheral lymphocytes, whereas most of the CD97 antigen presented on thyrocytes and C 643 cells had glycosylation sites resembling those of lymphocytes. The data suggest that CD97 expression may be a sensitive marker of dedifferentiation and of lymph node involvement in human thyroid tumors.