Exemestane in Treating Postmenopausal Women With Resected Stage I, Stage II, or Stage IIIA Breast Cancer Who Have Completed 5 Years of Tamoxifen

DISEASE CHARACTERISTICS: * Histologically confirmed invasive adenocarcinoma of the breast at time of original diagnosis * Tumor confined to the breast and ipsilateral axilla on clinical exam (T1-3, N0-1, M0) (stage I-IIIA) * No ipsilateral lymph nodes clinically fixed to one another or to other structures (N2 disease) at time of original diagnosis * Prior surgical resection, including 1 of the following: * Total mastectomy and axillary dissection (modified radical mastectomy) OR * Lumpectomy and axillary dissection * Prior post-lumpectomy breast radiotherapy required * Prior sentinel node biopsy allowed with the exception of the following: * If any sentinel node is histologically positive by hematoxylin and eosin (H \& E) OR histologically suspicious on H \& E and confirmed positive by immunohistochemistry then the patient must have a complete axillary dissection * Previously resected margins must be clear of invasive tumor and ductal carcinoma in situ * Currently disease free * Previously treated with tamoxifen for 57-66 months * Completed tamoxifen within the past 180 days * No bilateral malignancy or mass in the opposite breast suspicious for malignancy unless biopsy proven negative * No local, regional, or distant recurrence of disease or second primary breast malignancy (including contralateral breast cancer) * No advanced disease at time of original diagnosis (e.g., ulceration, erythema, infiltration of the skin or underlying chest wall \[complete fixation\], peau d'orange, or skin edema of any magnitude) * Tethering or dimpling of the skin or nipple inversion allowed * Current skeletal pain allowed if a bone scan and/or radiologic exam is negative for metastatic disease * Hormone receptor status: * Primary tumor estrogen receptor (ER) positive AND/OR * Progesterone receptor positive * Borderline ER positive tumors allowed if previously treated with tamoxifen PATIENT CHARACTERISTICS: Age: * Postmenopausal Sex: * Female Menopausal status: * Postmenopausal, defined as 1 of the following: * Prior bilateral oophorectomy * Absence of spontaneous menstrual cycle for more than 1 year * Follicle-stimulating hormone within the postmenopausal range if under 55 and had a prior hysterectomy without a bilateral oophorectomy Performance status: * Not specified Life expectancy: * At least 10 years Hematopoietic: * Absolute granulocyte count at least 1,500/mm\^3 * Platelet count at least 100,000/mm\^3 * Hemoglobin normal Hepatic: * Bilirubin less than 2 times upper limit of normal (ULN) * SGOT or SGPT no greater than 2.5 times ULN * Alkaline phosphatase less than 2 times ULN * No systemic hepatic disease that would preclude study participation Renal: * Creatinine no greater than 1.5 times ULN * No systemic renal disease that would preclude study participation Cardiovascular: * No systemic cardiovascular disease that would preclude study participation Other: * No other malignancy within the past 5 years except effectively treated squamous cell or basal cell skin cancer, surgically treated carcinoma in situ of the cervix, or surgically treated lobular carcinoma in situ of the ipsilateral or contralateral breast * No nonmalignant systemic disease that would preclude study participation * No psychiatric or addictive disorder that would preclude informed consent * Not pregnant PRIOR CONCURRENT THERAPY: Biologic therapy: * Not specified Chemotherapy: * Prior adjuvant chemotherapy allowed if administered concurrently with or prior to tamoxifen Endocrine therapy: * See Disease Characteristics * No concurrent estrogen, progesterone, clomiphene, testosterone, gonadotropin-releasing hormone, oral contraceptives, selective estrogen-receptor modulators, or dehydroepiandrosterone (DHEA) * Estring or estrogen vaginal cream at 0.3 mg or 1/8 of an applicator applied vaginally 3 times a week allowed * No other concurrent systemic hormonal therapy (e.g., tamoxifen or raloxifene) Radiotherapy: * See Disease Characteristics * Prior post-mastectomy loco-regional radiotherapy or post-lumpectomy regional radiotherapy allowed Surgery: * See Disease Characteristics Other: * Prior participation in other adjuvant NSABP study allowed if study was reported in peer-review publication or tamoxifen was not a study drug * Concurrent bisphosphonates or calcitonin for prevention or treatment of osteoporosis allowed * Concurrent statins (simvastatin, pravastatin, fluvastatin, atorvastatin, or lovastatin) or other drugs to control lipid levels allowed