Letrozole or Tamoxifen in Treating Postmenopausal Women With Breast Cancer

DISEASE CHARACTERISTICS: * Histologically confirmed resectable adenocarcinoma of the breast * pT1, pT2, pT3, or minimal dermal involvement on pathology only * pN0, pN1, pN2, or M0 * Negative nodal status * At least 8 nodes are negative * Unknown nodal status * Less than 8 nodes examined and no pathological finding * Positive nodal status * Any positive finding independent of the number of nodes examined * Negative sentinel node or no prior nodal dissection allowed if all other criteria met * Must have had total mastectomy, lumpectomy, or quadrantectomy * Should have prior chest wall radiotherapy after segmental mastectomy or histopathologic T4 dermal involvement * Stage I, II, or IIIa allowed if the tumor is completely removed macroscopically and margins of the resected tumor are microscopically free of tumor * Must undergo chest wall radiotherapy or second resection if microscopic disease at the mastectomy margins * No bilateral disease except in situ disease, either ductal or lobular of the contralateral breast * Postmenopausal * Regardless of prior hormonal replacement therapy (HRT) or hysterectomy: * Bilateral oophorectomy and any age * Radiologic castration and amenorrheic for at least 3 months and any age * Not postmenopausal at the start of adjuvant chemotherapy AND and completed at least 6 courses of prior cyclophosphamide, methotrexate, and fluorouracil (CMF) or at least 4 courses of prior anthracycline-cyclophosphamide continuation therapy and at least age 45 with follicle stimulating hormone (FSH), luteinizing hormone (LH), and estradiol (E2) postmenopausal levels * No prior HRT: * Prior hysterectomy and less than age 55 with FSH/LH/E2 postmenopausal levels * Prior hysterectomy and at least age 55 * No prior HRT or hysterectomy: * Amenorrhea more than 1 year and less than age 50 * Amenorrhea more than 6 months and at least age 50 * Prior HRT regardless of hysterectomy: * At least 1 month since prior HRT and less than age 55 with FSH/LH/E2 postmenopausal levels * At least 1 month since prior HRT and at least age 55 * FSH/LH/E2 postmenopausal levels and uncategorized * No distant metastases, including bone scans showing hot spots unconfirmed as benign disease or skeletal pain of unknown cause * At least 10% hormone receptor-positive tumor cells * Hormone receptor status: * Estrogen receptor positive AND/OR * Progesterone receptor positive PATIENT CHARACTERISTICS: Age: * 30 and over Sex: * Female Menopausal status: * Postmenopausal Performance status: * Not specified Life expectancy: * Not specified Hematopoietic: * WBC greater than 3,000/mm\^3 * Platelet count at least 100,000/mm\^3 * Hemoglobin greater than 10 g/dL Hepatic: * Bilirubin less than 3.0 mg/dL * SGOT or SGPT less than 1.5 times upper limit of normal * No hepatic disease that would preclude study Renal: * Creatinine less than 1.8 mg/dL * No renal disease that would preclude study Cardiovascular: * No cardiovascular disease that would preclude study * Prior deep vein thrombosis allowed if medically stable Pulmonary: * No lung embolism Other: * No other prior or concurrent malignancy within the past 5 years except adequately treated basal or squamous cell skin cancer or carcinoma in situ of the cervix * No prior noncompliance to medical regimens * No other nonmalignant systemic diseases that would preclude follow-up * HIV negative PRIOR CONCURRENT THERAPY: Biologic therapy: * Prior immunotherapy or biological response modifiers (e.g., interferon) allowed Chemotherapy: * See Disease Characteristics * Prior adjuvant or neoadjuvant chemotherapy allowed * Concurrent adjuvant chemotherapy allowed Endocrine therapy: * See Disease Characteristics * Prior neoadjuvant hormonal therapy allowed (e.g., antiestrogens, progestins, or aromatase inhibitors) if no more than 4 months duration and no disease progression * Prior corticosteroids allowed * At least 4 weeks since prior HRT * Prior adjuvant antiestrogen therapy allowed if less than 1 month duration and immediately after surgery, radiotherapy, and/or chemotherapy * Prior antiestrogens for chemoprevention allowed if at least 18 months between completion of chemoprevention and diagnosis * No other concurrent antiestrogens or aromatase inhibitors * No concurrent raloxifene * No concurrent systemic HRT with or without progestins of more than 3 months duration Radiotherapy: * See Disease Characteristics * Concurrent radiotherapy allowed Surgery: * See Disease Characteristics Other: * At least 30 days since prior systemic investigational drugs * At least 7 days since prior topical investigational drugs * Concurrent bisphosphonates allowed