Inclusion Criteria:

          -  REGISTRATION-INCLUSION CRITERIA

          -  Disease characteristics:

               -  Histological confirmation of invasive breast carcinoma

               -  Stage I-III breast cancer

               -  Estrogen receptor (ER) positive disease according to American Society of Clinical
                  Oncology (ASCO)/College of American Pathologists (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

               -  Neoadjuvant and/or adjuvant chemotherapy, if needed

          -  Post-menopausal defined as

               -  Age >= 60 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

          -  Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0, 1, or 2

          -  Hemoglobin >= 8.0 g/dL (obtained =< 14 days prior to registration)

          -  Absolute neutrophil count (ANC) >= 1500/mm^3 (obtained =< 14 days prior to
             registration)

          -  Platelet count >= 70,000/mm^3 (obtained =< 14 days prior to registration)

          -  Total bilirubin =< 1.5 x upper limit of normal (ULN) (obtained =< 14 days prior to
             registration)

          -  Alanine aminotransferase (ALT) and aspartate transaminase (AST) =< 3 x ULN (obtained
             =< 14 days prior to registration)

          -  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)

          -  RE-REGISTRATION-INCLUSION CRITERIA

          -  Confirmation that baseline blood sample was drawn and submitted

          -  Blood estrogen levels after cycle 1 anastrozole (ANA1) must meet the following
             criteria:

               -  E1 >= 1.3 pg/ml, AND

               -  E2 >= 0.5 pg/ml

        Exclusion Criteria:

          -  REGISTRATION-EXCLUSION CRITERIA

          -  Pre-menopausal women receiving ovarian function suppression (goserelin, leuprolide,
             etc.)

          -  Stage IV (metastatic) breast cancer

          -  HER2 positive breast cancer as defined by

               -  HER2 immunohistochemistry (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 (1 mg 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 ductal carcinoma in situ (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 selective estrogen receptor
             modulator (SERM). Exception: Therapy with a SERM (tamoxifen or raloxifene) is allowed
             if patient has been off treatment for >= 6 months prior to registration