DISEASE CHARACTERISTICS:

* One of the following histologically proven diagnoses:

* Primary invasive adenocarcinoma of the unilateral or bilateral breast

* Stage I, IIA, or IIB (T1-3, N0-1, M0)
* No recurrent invasive breast cancer
* Ductal carcinoma in situ (DCIS)
* Lobular carcinoma in situ (LCIS) with microinvasion
* Paget's disease of the nipple
* No sarcoma, lymphoma, or apocrine, adenocystic, or squamous cell cancer of the breast
* Currently free of breast cancer (no evidence of disease)

* No evidence of distant disease on chest x-ray or chest CT scan and mammogram of the opposite breast within the past year
* Prior definitive local treatment of primary lesion (mastectomy or breast-sparing procedure with radiotherapy) and either axillary node or sentinel node biopsy

* Surgical margins clear of both infiltrating carcinoma (any type) and DCIS

* No gross or microscopically positive margins except:

* Invasive cancer or DCIS at the focal margin treated with definitive radiotherapy
* Gross or LCIS at the final margin
* Biopsy requirement waived for DCIS or LCIS with minimal microinvasion
* Patients with breast-sparing procedure must have received or be planning to receive radiotherapy at start of tamoxifen treatment
* No endometrial simple or cystic hyperplasia, proliferative changes, complex (adenomatous) or atypical hyperplasia, or carcinoma
* Patients must be planning one of the following:

* Starting adjuvant tamoxifen for five years OR
* Started tamoxifen within 28 days prior to study and planning to receive adjuvant tamoxifen for five years
* Hormone receptor status:

* Candidate for adjuvant tamoxifen therapy

PATIENT CHARACTERISTICS:

Age:

* Adult

Sex:

* Female

Menopausal status:

* Postmenopausal defined as:

* At least 1 year since last menstrual period
* At least 2 months since bilateral oophorectomy prior to breast cancer diagnosis
* 4-12 months since last menstrual period and FSH elevated to postmenopausal range
* Postmenopausal estrogen therapy and 55 years of age or older

Performance status:

* Not specified

Life expectancy:

* Not specified

Hematopoietic:

* Not specified

Hepatic:

* Not specified

Renal:

* Not specified

Other:

* Fertile patients must use effective contraception during and for at least 2 months after study
* No other malignancy within the past 5 years except adequately treated basal cell or squamous cell skin cancer, carcinoma in situ of the cervix, or stage I or II cancer currently in complete remission
* No concurrent nonmalignant-related illness that would preclude study

PRIOR CONCURRENT THERAPY:

Biologic therapy:

* Not specified

Chemotherapy:

* Adjuvant chemotherapy allowed
* No concurrent chemotherapy

Endocrine therapy:

* See Disease Characteristics
* No prior hormonal treatment for breast cancer (except tamoxifen)
* No concurrent postmenopausal estrogen therapy

Radiotherapy:

* See Disease Characteristics

Surgery:

* See Disease Characteristics
* No prior or concurrent hysterectomy

Other:

* No prior or current participation in an adjuvant intergroup trial