The regulation of estrogen activity through the formation and cleavage of sulfoconjugates of estrogens is known to be related to the progression and metastasis of estrogen-dependent breast carcinomas, but the involvement of sulfoconjugates in the steroid stimulation of endometrial functions and the progression of endometrial adenocarcinomas is not clearly understood yet. Estrogen sulfotransferase (EST) in the uterine endometria during the follicular phase was more active than during the luteal phase, but estrogen sulfate (ES) sulfatase exhibited lower activity during the follicular phase than during the luteal phase. However, ES sulfatase activities in cancerous tissues were lower than those in normal endometria and endometrial adenocarcinoma-derived cells, among which the activity was exceedingly high in Ishikawa cells, suggesting that ES sulfatase in Ishikawa cells contributes to the estrogen-dependent growth of these cells. EST Steroid receptors in the endometrial epithelium are known to be involved in the expression of several genes in response to steroid stimulation of endometrial functions related to the implantation of fertilized eggs, and a failure of the steroid-mediated signal transduction pathway is implicated in the progression of endometrial adenocarcinomas, [2][3][4] as well as of breast carcinomas.5-7) Estrogen is a steroid relevant to the progression of transformed cells, but its active form, 17β-estradiol (E2), is locally generated by several enzymes, such as estrogen sulfotransferase (EST), estrogen sulfate (ES) sulfatase, aromatase, 17β-hydroxysteroid dehydrogenase and so on, since the circulating estrogen in human sera comprises inactive estrone (E1) sulfate.8) Among these enzymes, EST and ES sulfatase in breast carcinomas have been shown to be involved in the regulation of estrogen activity by forming and cleaving the sulfoconjugate of estrogen to inhibit and promote the binding of estrogen to its receptor, respectively. 9, 10) In this connection, inhibition of sulfotransferase by several chlorinated phenol derivatives, such as 6,6-dichloro-4-nitrophenol and hydroxylated polychlorinated biphenyl, has been shown to enhance the activity of endogenous estrogen, and to be probably implicated in human reproductive disorders and an increased incidence of breast carcinomas. 11) On the contrary, environmental xenoestrogens, such as bisphenol, 4-n-octylphenol, 4-n-nonylphenol, and 17α-ethynylestradiol, which have became ubiquitous in the environment and have been shown to cause a decrease in sperm quality and progression of estrogen-dependent cancers by interacting with the estrogen receptor, are converted to their sulfoconjugates by cytosolic sulfotransferase, resulting in detoxification of the chemicals through abolition of their binding ability to the receptor and in facilitation of their removal from the body through an increase in water solubility.12, 13) Thus, the sulfoconjugation of estrogens and related compounds occurring inside cells plays essential roles in the regulation of estrogen rec...