It is well known that sex steroids are closely involved in the growth of human breast carcinomas, and the great majority of breast carcinomas express sex steroid receptors. In particular, recent studies have demonstrated that estrogens and androgens are locally produced and act in breast carcinoma tissues without release into plasma. Blockade of intratumoral estrogen production potentially leads to an improvement in the prognosis of invasive breast carcinoma patients, and, therefore, it is important to obtain a better understanding of sex steroid-producing enzymes in breast carcinoma. In this review, we summarize recent studies on tissue concentration of sex steroids and expression of enzymes related to intratumoral production of estrogens [aromatase, steroid sulfatase (STS), and 17beta-hydroxysteroid dehydrogenase type 1 (17betaHSD1)], and androgens (17betaHSD5 and 5alpha-reductase) in invasive and in situ (noninvasive) breast carcinomas, and discuss the significance of intratumoral production of sex steroids in breast carcinoma.