Inclusion Criteria - Registration:

• Women of age ≥ 18 years.
• Histological confirmation of invasive breast carcinoma.
• Stage I-III breast cancer
• Estrogen receptor (ER) positive disease according to ASCO/CAP guidelines as ER≥1% positive nuclear staining.
• Completion of all planned cancer treatments prior to registration:
  • surgical resection of breast and nodal surgery;
  • NOTE: Reconstructive surgery does not have to be completed
  • adjuvant radiation therapy, if needed; and
  • neoadjuvant and/or adjuvant chemotherapy, if needed.
• Post-menopausal defined as
• Age ³60 and amenorrhea >12 consecutive months OR
• Previous bilateral oophorectomy OR
• Age <60 and amenorrhea >12 consecutive months and documented follicle stimulating hormone (FSH) level within post-menopausal range according to institutional standard
• NOTE: Patients who did not meet these criteria at time of diagnosis and received pre-operative (neoadjuvant) or post-operative (adjuvant) chemotherapy will not be allowed to participate.
• ECOG Performance Status (PS) 0, 1, or 2 .
• The following laboratory values obtained ≤14 days prior to registration:
  • Hemoglobin ≥ 8.0 g/dL;
  • Absolute neutrophil count (ANC) ≥ 1500/mm^3;
  • Platelet count ≥ 70,000/mm^3;
  • Total bilirubin ≤ 1.5 x ULN;
  • Alanine aminotransferase (ALT) and aspartate transaminase (AST) ≤ 3 x ULN.
• Ability to swallow oral medication.
• Provide written informed consent.
• Willingness to provide mandatory blood specimens for correlative research.
• Willing to return to enrolling institution for follow-up (during the Active Monitoring Phase of the study).

Exclusion Criteria - Registration:

• Pre-menopausal women receiving ovarian function suppression (goserelin, leuprolide, etc.).
• Stage IV (metastatic) breast cancer.
• HER2 positive breast cancer as defined by:
  • HER2 IHC ≥ 3+;
  • HER2/CEP17 ≥ 2.0;
  • HER2/CEP17 < 2.0 and average HER2 copy number of ≥ 6.0. signals/cell
• Prior endocrine therapy for this breast cancer.
• Exceptions:
  • Pre-operative aromatase therapy (anastrozole, letrozole, or exemestane)  and last treatment was ≥ 4 weeks prior to registration; OR
  • Pre-operative tamoxifen therapy and last treatment was ≥ 12 weeks prior to registration.
• Currently receiving any of the following cancer-directed therapies:
  • Radiation therapy;
  • Systemic therapy such as chemotherapy (standard or investigational);
  • Bisphosphonate therapy started < 4 weeks prior to registration.
• NOTE: If patient is currently on bisphosphonate therapy she must be on stable dose for ≥ 4 weeks prior to registration.
• Patients not currently taking bisphosphonates will be allowed to start bisphosphonate therapy after completion of anastrozole (1mg and 10 mg daily (if given)). Information regarding bisphosphonate therapy will be collected.
• Current use of systemic or topical exogenous estrogen or progesterone (menopausal hormone replacement therapy [HRT]).
• Prior ovarian function suppression (leuprolide, goserelin, etc).
• Inability to provide informed consent.
• History of contralateral DCIS or invasive breast cancer.
• NOTE: Exception allowed if:
  • Patient did not receive adjuvant endocrine therapy; OR
  • Patient received adjuvant endocrine therapy but has been off treatment for at least 6 months prior to registration.
• Concurrent active malignancy or history of malignancy ≤ 3 years prior to registration.
• NOTE: Exceptions allowed for successfully treated cervical carcinoma in situ, lobular carcinoma in situ of the breast, papillary thyroid cancer, or non-melanoma skin cancer.
• Prior prevention therapy with an aromatase inhibitor or a SERM.
• Exception: Therapy with a SERM (tamoxifen or raloxifene) is allowed if patient has been off treatment for ≥ 6 months prior to registration.