Inclusion Criteria:

- Women 18 years of age or older

- History of invasive ER+ or PR+ breast cancer treated with at least 4.5 years of
tamoxifen

- No current evidence of recurrent invasive disease or metastatic disease. Patients
may have a history of bilateral breast cancer

- Premenopausal (estradiol level in premenopausal range, >20pg/ml, within the prior 28
days)

- Liver function tests and creatinine <2.5 times the upper limit of normal within the
28 days prior to enrollment

- ECOG Performance Status 0-1

- Must agree to use non-hormonal contraception (condoms, diaphragm, IUD, sterilization,
abstinence, etc) and no other hormonal therapy during trial and until 3 months after
letrozole is stopped

- Negative pregnancy test within 14 days prior to enrollment

- Patient must be able to speak, read and write in English

Exclusion Criteria:

- Previous treatment with an oral or IV bisphosphonate in the prior two years

- History of cancer other than breast cancer within 5 years excluding basal/squamous
cell skin carcinoma in situ of the cervix

- Women with evidence of current local recurrence or metastatic breast cancer

- Pregnant women

- Nursing women

- Women who are currently taking tamoxifen and are unwilling to stop this medication

- Women with a known deleterious BRCA 1 or BRCA 2 mutation