Institut für Molekularbiologie und Biochemie, Frauenklinik Charlottenburg, Freie Universität Berlin, Germany (Received 15 April 1975) Recent results obtained by Tseng & Gurpide (1974) and Pollow, Lübbert, Boquoi, Kreuzer, Jeske & Pollow (1975 a) have shown that oestradiol is oxidized about ten times more rapidly by normal human endometrium of the secretory phase than by tissue of the proliferative phase. This possibly means that progesterone regulates (by way of a receptor mechanism?) the activity of the 17β-hydroxysteroid dehydrogenase (17β-HSD) which catalyses the oxidation of oestradiol in the endometrial target cell. Since progestational agents play a definite role in the management of advanced adenocarcinoma of the endometrium the present study evaluates the effect of gestagen therapy upon the 17β-HSD activity in this malignancy. Normal endometrium (proliferative and secretory, 18 cases of each) was obtained by curettage. Endometrial adenocarcinomata (well, moderately well and undifferentiated, four cases of each) were obtained by curettage (before