Since first reports in the literatures by Mitwally and Casper, along the last two decades, the aromatase inhibitor letrozole has been found as a useful and safe agent for ovulation induction in anovulatory women, e.g., PCOS (Polycystic Ovarian Syndrome), ovarian superovulation in ovulatory women, e.g., unexplained infertility, and in conjunction with gonadotropins to achieve better ovarian response during assisted reproduction (1-5). This debate discusses if ultrasound is ‘not’ required during letrozole treatment ‘alone’ as an agent for ovarian stimulation. I am against not using ultrasound monitoring during letrozole treatment. I think ultrasound is required in patients undergoing letrozole treatment, at two points: a baseline ultrasound when starting administration and a follow-up ultrasound few days after finishing letrozole administration. Ultrasound during letrozole treatment may be highly recommended, advisable and least required in the following situations: Highly recommended for safety when ruling out possible underlying pregnancy and ovarian cysts; Advisable to determine stimulation protocol, and response to letrozole treatment, as well as timing HCG administration and fertility treatment including timing intercourse and IUI; Least required to determine the risk of multiple pregnancy which is pretty low with letrozole treatment.