Breast cancer is the most common cancer in US females and is the second leading cause of cancer death among women. By contrast, Asian women consuming a traditional diet high in soy products have a relatively low incidence of breast cancer. Asians who emigrate to the United States and adopt a Western diet lose this protection. Soy-based diets are high in phytoestrogens, and one of these components is genistein. Using the dimethylbenz(a)anthracene (DMBA) mammary cancer rodent model, we have investigated the breast cancer protective potential of genistein. Our results demonstrate that neonatal and prepubertal genistein treatments altered the ontogeny of the mammary gland and rendered the adult animals less susceptible to chemically-induced mammary cancer. Neonatal genistein treatment did not significantly alter the rate of formation and persistence of DMBA-DNA adducts in the mammary gland. While high concentrations of genistein during the neonatal period caused adverse effects on ovarian follicular development, prepubertal genistein treatment did not appear to be toxic in either the female reproductive tract or the endocrine system.