Letrozole  is an oral non-steroidal aromatase inhibitor for the treatment of hormonally-responsive breast cancer after surgery.

Letrozole has been used for ovarian stimulation by fertility doctors since 2001 because it has fewer side-effects than clomiphene  and less chance of multiple gestation. A study of 150 babies following treatment with letrozole or letrozole and gonadotropins presented at the American Society of Reproductive Medicine 2005 Conference found no difference in overall abnormalities but did find a significantly higher rate of locomotor and cardiac abnormalities among the group having taken letrozole compared to natural conception.A larger, follow-up study with 911 babies compared those born following treatment with letrozole to those born following treatment with clomiphene. That study also found no significant difference in the rate of overall abnormalities, but found that congenital cardiac anomalies was significantly higher in the clomiphene group compared to the letrozole group.

The anti-estrogen action of letrozole has been shown to be useful in pretreatment for termination of pregnancy, in combination with misoprostol. It can be used in place of mifepristone, which is expensive and unavailable in many countries.

Letrozole is sometimes used as a treatment for gynecomastia, although it is probably most effective at this if caught in an early stage (such as in users of anabolic steroids).

Some studies have shown that letrozole can be used to promote spermatogenesis in male patients suffering from nonobstructive azoospermia.

Letrozole has also been shown to delay the fusing of the growth plates in mice.When used in combination with growth hormone, letrozole has been shown effective in one adolescent boy with a short stature.

Letrozole has also been used to treat endometriosis.