Tenascin (TN) is an extracellular matrix glycoprotein (ECM) that participates in embryogenesis and carcinogenesis. The aim of this study was to investigate immunohistochemically the expression of TN in the normal, hyperplastic, and neoplastic endometrium (endometrial adenocarcinoma). In the adenocarcinomas, the results were correlated with patient age, menopausal status, stage, grade, myometrial invasion, and vascular invasion. TN expression was studied in the following cases: proliferative endometrium (10 cases), early secretory endometrium (10), secretory endometrium (10), simple hyperplasia (15), complex hyperplasia (15), atypical hyperplasia (15), and endometrial adenocarcinomas (25). Staining of basal membranes and the cytoplasm of the stromal and epithelial cells was evaluated semiquantitatively. Positive staining was observed in the vascular and glandular basal membranes, stromal cells, and epithelial cells of proliferative, hyperplastic, and neoplastic endometrium. The difference in percentage of stained stromal cells between the neoplastic and the nonneoplastic (proliferative and hyperplastic) endometrium was significant (p<0.005). However, the percentage of stained epithelial cell area in hyperplasia was significantly higher than that of adenocarcinoma and functional endometrium (p<0.005). We conclude that TN is an extracellular matrix glycoprotein that plays a role in proliferation and possibly endometrial carcinogenesis.