OBJECTIVES: I. Compare the duration of overall survival and disease-free survival in premenopausal women with operable, high risk node negative or axillary node-positive breast cancer who have undergone complete surgical resection of all known disease by means of total or partial mastectomy, and have received standard adjuvant chemotherapy with cyclophosphamide, methotrexate, and fluorouracil (CMF), cyclophosphamide, epirubicin, and fluorouracil (CEF), or doxorubicin and cyclophosphamide (AC) followed by either daily tamoxifen for 5 years or placebo. II. Compare the short- and long-term toxicity in patients receiving tamoxifen versus placebo. III. Monitor follicle-stimulating hormone, luteinizing hormone, and estradiol levels, and determine whether overall survival and disease-free survival are affected by hormonal or menopausal status during or at completion of adjuvant chemotherapy or during or after tamoxifen or placebo treatment in these patients.