We investigated spontaneous mammary tumor development and mammary gland morphogenesis in female Tg.NK mice postnatally exposed to dietary soy isoflavones (0, 11, 39, and 130 mg aglycones/kg diet) added to a Western-style diet. Instead of preventing mammary tumorigenesis, the highest dose of isoflavones was associated with a small but significant increase in the number and size of tumors as compared to mice administered a Western-style control diet (P < 0.05). At postnatal Week 6, dynamic activity (measured as apoptotic density) at the highest dose and the degree of branching of the mammary tree in all isoflavone-exposed groups was increased as compared to controls (P < 0.05). At adulthood, the epithelium appeared more quiescent in the medium- and high-dose groups evident by reduced apoptotic density and a reduction in the percentage of terminal end buds (TEBs), respectively, as compared to controls (P < 0.05). The number of actively dividing cells within the TEBs was unaffected by isoflavone exposure as was the activity of drug-metabolizing and antioxidant enzymes. In conclusion, isoflavones may augment mammary gland and mammary tumor development.