The US Food and Drug administration has approved the drug letrozole ( an aromatase inhibitor) for the extended adjuvant treatment of post-menopausal women with breast cancer, who have received five years of tamoxifen therapy. The FDA based its approval on the MA-17 trial data. Approximately one sixth of oestrogen positive breast cancer patients suffer a relapse of their disease at greater than five years from presentation. Although tamoxifen has reduced the chance of such a relapse, its continued usefulness after five years is insignificant and late problems may occur (e.g. on the endometrium). "Extended" adjuvant therapy with letrozole is the first adjunctive therapy to effectively reduce these ongoing risks of breast cancer recurrence. This is perceived to be an important advance.