In situ estrogen metabolism and synthesis have been considered to play a very important role in the development and progression of human endometrial carcinoma. 17Beta-hydroxysteroid dehydrogenases (17-HSDs) are enzymes involved in the formation of active sex steroids, including testosterone, estrone (E1) and estradiol (E2). Estrogens are interchanged by two enzymes, 17-HSD types 1 and 2, type 1 converts E1 to E2, and type 2 does reverse actions. 17-HSD type 5 catalyzes the reduction of androstenedione to testosterone. 17-HSD type 2 expression was decreased through normal endometrium, hyperplasia and carcinoma accordingly. There was a significant inverse correlation between intratumoral E2 concentration and the level of 17-HSD type 2 mRNA in endometrial carcinoma. 17-HSD type 5 expression was significantly increased through normal endometrium, hyperplasia and carcinoma accordingly. These results indicated that 17-HSD types 2 and 5 play an important role in the regulation of in situ estrogen production in endometrial carcinoma.