 Letrozole is a potent and highly selective third generation aromatase inhibitor that was developed in the early 1990s. Aromatase is an enzyme of the cytochrome P-450 superfamily and the product of the CYP19 gene, which catalyzes the reaction that converts androgenic substances to estrogens in many tissues, including the granulosa cells of ovarian follicles. Letrozole competitively inhibits the activity of the aromatase enzyme and has a half-life of approximately 48 hours. Because of its potent, sustained suppression in the plasma levels of estradiol, this drug has been recently found to be superior to tamoxifen in the treatment of advanced-stage post-menopausal breast cancer. Conventional ovarian stimulation often results in very high estrogen levels. Since the high estrogen levels are often unsafe for breast cancer patients, fixed dose aromatase inhibitor protocols with Letrozole were developed, to achieve effective ovarian stimulation with reduced estrogen levels to prevent tumor progression and short-term cancer recurrence. 