At present, the most widely used medications for controlled ovarian hyper-stimulation (COH) include clomifene citrate (CC) and gonadotropins. CC is a safe, effective and cheap agent for COH; however, antiestrogenic effects on endometrium and cervical mucous may prevent pregnancy despite successful ovulation. Failure of CC usually warrants gonadotropins, which are associated with high cost, parenteral administration, high risk of ovarian hyper-stimulation syndrome and multiple pregnancy. In these circumstances, aromatase inhibitors (Als) should be considered as an alternative regimen with or without exogenous gonadotropins in patients who failed to ovulate with CC. Absence of persistent antiestrogenic effect due to their short half-life, lack of estrogen receptor depletion and promising outcome in patients with poor ovarian response are the prominent advantages. This review is aimed at providing insight to the chemical structure and pharmacokinetics of Als and their (in particular, letrozole) clinical utility in hyper-stimulated or in vitro fertilization cycles. The potential teratogenic effects are also discussed.