According to the findings, it can, therefore, be said that breast cancer, as well as ovarian cancer, is a large problem of medicine and both are life treating, especially for women. These cancers are of particular importance because they are promoted by estrogen. In 2000, approximately 130,000 women died of breast cancer. nearly two-thirds of these women were postmenopausal. In pre-menopausal women estrogen is produced in the ovaries, so reducing estrogen from other sources is not important, but in postmenopausal women, most of the estrogen in the body is produced in the adrenal gland from the conversion of androgens and also from adipose tissue through a special enzyme called aromatase. The large influence of the estrogen produced outside the ovaries by aromatization is clear. In postmenopausal women, their estrogen comes from the actions of aromatase-final step in the pathway of estrogen biosynthesis by creating an aromatic ring in the steroid molecule, and so its inhibitor may be used to treat estrogen tumors, and so, effective treatment in breast cancer is an aromatase inhibitor. Also, it is used in women when their ovaries are no longer producing estrogen. It is not the same in premenopausal women or the opposite effects appear, the main estrogen comes up from ovaries, not by conversion androgen to estrogen, so blocking aromatization effects does not increase the androgen production. According to clinical researches, a correlation has been establishing between estrogen receptors and the growth of breast cancer and so effective therapy can be made by aromatase inhibitor working more efficient then tamoxifen. The hormone estrogen delivers growth thru signals to the hormone receptors, so less estrogen in the body leads to fewer growth signals and stopping the growth of cancer. Now medical experts consider aromatase inhibitor for postmenopausal women with invasive hormone receptor positive for breast cancer, in the early and advanced stage. Aromatase inhibitor, according to research, can stimulate ovulation- can treat infertility. According to 65 patients included in the studies, drugs can suppress estrogen production in endometriosis and reduces pain and chance of endometriosis in combination with hormonal treatment. It is significant to mention that there should be no long-term use because it may cause bone loss. It also helps bodybuilders, who take anabolic steroids from being converted to estrogen such as in the case of gynecomastia. In one recent study, however, aromatase inhibitors were found to be no more successful at treating pubertal gynecomastia than a placebo. Aromatase inhibitors have also been shown to reverse age-related declines in testosterone, as well as primary hypogonadism. The third generation drugs: anastrozole, letrozole, and exemestane have given remarkable specificity and potency. Leading thru initial results from clinical trials, these agents will become the cornerstones of future endocrine therapy as a successful rational drug. It is important that the aromatase inhibitor have additional benefits thru pleiotropic effects as a particular role in preventing cancer or treating benign conditions.