It has been demonstrated that biologically active estrogens are locally produced from circulating inactive steroids in an intracrine mechanism in the breast carcinoma. The in situ production of estrogens is considered to play an important role in the proliferation of breast cancer cells, especially in the postmenopausal women. Therefore, the total blockade of this pathway may lead to an improvement in the prognosis in breast cancer patients due to the inhibition of estrogenic actions. In this review, we describe the recent studies of enzymes related to intracrine mechanism of estrogen synthesis, including aromatase, steroid sulfatase (STS), and 17beta-hydroxysteroid dehydrogenase, in human breast carcinoma tissues, and discuss the biological significance of local production of estrogens in human breast cancer.