The purpose of this study is to determine the safety and tolerability of the combination of two drugs (letrozole and leuprolide) in women who have already taken tamoxifen for at least 4.5 years. Letrozole, an aromatase inhibitor (which blocks an enzyme that produces estrogen), is a drug that is FDA approved. It has been shown to reduce the risk of breast cancer recurrence in postmenopausal women with breast cancer who have been previously treated with tamoxifen. Letrozole works by stopping the production of estrogen in parts of the body other than the ovaries. Leuprolide is a drug that stops a women's ovarian cycles. This process is known as ovarian function suppression. Stopping a women's menstrual cycle may be effective against breast cancer for some patients when given as initial therapy. The combination of letrozole and leuprolide is considered a standard treatment for women with metastatic breast cancer, and is also sometimes used for treatment of premenopausal early stage breast cancer, but it has not been accepted as a standard of care treatment.