Letrozole is a reversible (Type II), nonsteroidal aromatase inhibitor. Aromatase catalyzes the final and rate-limiting step in the conversion of androgens to estrogens in peripheral tissues. This occurs mainly in adipose tissue, but also in normal and malignant breast tissues, and provides the main source of estrogen in postmenopausal women. The goal of hormone therapy in breast cancer is to deprive tumour cells of estrogens, which are implicated in the development or progression of tumours.3,4 Maximal estrogen suppression is produced by a 0.1 mg dose,3,5 although a higher dose (ie, 2.5 mg per day) was associated with increased clinical responses.3 Maximal estrogen suppression occurs 48-78 hours after a single dose.6 Highly selective blockade of aromatase does not interfere with the production of other steroids (eg, adrenal corticosteroids, aldosterone)7,8 or thyroid stimulating hormone.5 Letrozole does not have progestogenic, androgenic or estrogenic activity.7,8 Differences in the mechanism of action may contribute to the apparent lack of cross-resistance between steroidal (eg, exemestane) and nonsteroidal (eg, anastrozole, letrozole) aromatase inhibitors.9