Abstract: To reduce the risk of breast cancer, a drug is periodically administered to young mammals before pregnancy in doses of about 1 microgram to 50 micrograms per kilogram of body weight. The drug: (1) competes with and displaces estradiol 17 beta from the mammary gland cells in an effective manner to prevent the possible formation of DNA-damaging epoxide estradiol metabolites; (2) binds to breast tissue to a greater extent than estradiol 17 beta; (3) induces terminal nonlactation differentiation of the mammary gland; (4) is nontoxic and noncarcinogenic; and (5) preferably does not cause anti-ovulatory activity. The drug is selected from a group of drugs including: (1) 4-OH estradiol; (2) d-equilenin; and (3) 17 alpha ethynyl estriol.