To facilitate our understanding of estrogen participating in the genesis of human endometrial hyperplasia and carcinoma and the influence of the surrounding mesenchyme in their development, we assayed estrogen receptors in the epithelium and stroma of various human endometrial tissues by immunocytochemistry. The receptors were localized in fresh frozen sections using the ER-ICA kits (Abbott Laboratories, North Chicago, Illinois). The receptor content was highest in the proliferative endometrium and decreased gradually throughout the postovulatory phase of the menstrual cycle in both the epithelium and the stroma and this occurred more rapidly in the stroma. Estrogen receptor content was high in both the epithelium and the stroma of hyperplasia and was thus similar to that of the proliferative endometrium, the receptor content was low in the epithelium of endometrial intraepithelial neoplasia (hyperplasia with cytologic atypia) closely resembling that of carcinoma. In carcinomas there was a heterogenous distribution of estrogen receptors both in the epithelium and the stroma, but in general, the receptor content was low as compared with normal proliferative or hyperplastic endometrium; however, in selected cases the content of estrogen receptor in the stroma was high, despite the lack of receptors in the epithelium.