Among various tumors, steroid hormone dependency is a hallmark of endometrial carcinoma. In this review article, we describe the estrogen-induced growth mechanism of normal and malignant endometrial cells with respect to the involvement of cell cycle regulators, and demonstrate the pivotal role of estrogen-induced cyclin D1 up-regulation. We also discuss the role of estrogen in the pathogenesis of endometrial carcinoma in relation to the regulation of mismatch repair (MMR) activity. We propose a new aspect of high serum estrogen levels as a possible suppressor of endometrial carcinogenesis via increasing MMR activity.