Endometrial estrogen and progestin receptors were quantitatively measured in the cytosol (ERc, FRc) and nuclear (ERn, PRn) fractions, the activity of 17β-hydroxysteroid dehydrogenase measured in 13 normal women in the late prolif erative phase of the cycle (control group), in 33 patients with adenocarcinoma, and in 6 patients with other malignancies of the endometrium. The parameters measured had relatively small variations in the control group, whereas the opposite was true for the malignant endometrium. ERc and FRc were present in significantly higher concentrations in normal endometrial tissue (167 and 1697 fmol/mg cytosol protein, respectively) than in malignant endometrial tissue (45 and 116 fmol/mg cytosol protein, respectively), and the ratios of ERc/ERn and PRc/PRn were higher (P<<.001 in both cases) in the normal group. The activities of 17β- hydroxysteroid dehydrogenase were identical in normal and adenocarcinoma tissue and correlated with PRc in carcinomatous endometrium. The present results support previous findings that the great majority of endometrial adenocarcinoma specimens have significant concentrations of ERcand PRc and thatthese concentrations are lower than in normal endometrium. In addition, they demonstrate that nuclear location of the female sex steroid receptors is favored in the malignant tissue. Despite these differences, the 17β-hydroxysteroid dehydrogenase activities were identical in proliferative endometrium and in endometrial adenocarcinoma.