Women with hormone-receptor positive breast cancer are typically treated with hormone therapy as part of their treatment after surgery. In the past few years it has been found that treatment with aromatase inhibitors is superior to tamoxifen in postmenopausal women. Tamoxifen is still used for premenopausal women, however, because aromatase inhibitors are not effective in women who have functioning ovaries. Some women are premenopausal at the time they are diagnosed with breast cancer, but then stop having menstrual periods when they are treated with chemotherapy. It is unclear if these women can also be treated safely with aromatase inhibitors.