The expression patterns of CD44s and CD44v6 were immunohistochemically compared with those of normal, hyperplastic and malignant endometrium. In normal endometria (n=37), endometrioses (n=46) and adenomyoses (n=20), the surface and glandular epithelial cells were negative for CD44s and CD44v6 in a proliferative pattern and positive in a secretory pattern, whereas the stroma was only positive for CD44s in both proliferative and secretory patterns. The endometrial hyperplasia (4 simple and 9 complex) had the identical patterns with normal proliferative phase of endometrium. Only one case showing complex hyperplasia with atypia was focally positive for CD44s and CD44v6 in glandular epithelia. CD44s and CD44v6 were positive in all endometrial adenocarcinomas (13), except one CD44s-negative case. In summary, the expressions of CD44s and CD44v6 in endometriosis and adenomyosis recapitulated those of normal cyclic endometrium. The expression patterns in endometrial hyperplasia were similar to those in normal proliferative endometrium, whereas the endometrial adenocarcinoma showed abnormal expressions for CD44s and CD44v6. Thus it was considered that the ectopic endometrium in endometriosis and adenomyosis was not aberrant as in endometrial carcinoma on the aspects of immunohistochemical expressions of CD44s and CD44v6.