PATIENT CHARACTERISTICS: Age: 35 to 70 Sex: Women only Menopausal status: Pre-, peri-, or postmenopausal Performance status: Not specified Life expectancy: At least 10 years Cardiovascular: No prior deep vein thrombosis Pulmonary: No prior pulmonary embolus Other: Psychologically and physically able to undertake therapy for 5 years No other medical condition more serious than risk of breast cancer No prior cancer except: Nonmelanomatous skin cancer In situ cancer of the cervix No pregnant women Effective nonhormonal contraception required of fertile women