There are no practical options to reduce breast cancer risk in American women that are without side-effects. Almost all preventive strategies are designed to diminish the role of estrogens in promoting breast cell proliferation. Brassica vegetables (e.g., broccoli) contain indole glucosinolates that can shift estrogen metabolism away from the highly estrogenic 16α-hydroxyestrone (16HE) and toward 2-hydroxyestrone (2HE), which has little estrogenic activity on breast cells. The relative strength of these two pathways is measured in urine as the ratio 2HE/16HE (2/16). In controlled trials that have enrolled premenopausal women or young men, large quantities of Brassica vegetables or indole glucosinolate derivatives decreased urinary 16HE levels relative to 2HE levels, possibly lowering breast cancer risk. However, in order to be an acceptable and effective preventative, Brassica vegetables must shift estrogen metabolism as Brassica consumption is practiced within free-living women. The goal of this research is to determine if Brassica should be further explored as a strategy to prevent breast cancer. In order to do so, it was determined that three issues must be addressed: (1) Brassica consumption must increase the 2/16 ratio among healthy free-living women, without serious side-affects, (2) the 2/16 ratio must be a valid indicator of breast cancer risk, and (3) a reliable and valid method to estimate Brassica consumption must be identified. As part of this research, thirty-seven healthy postmenopausal women participated in a dietary intervention designed to facilitate daily Brassica consumption. The diet, 2/16, and other information were measured before, during, and after the intervention. The 2/16 ratio significantly increased with greater Brassica intake. However, urinary 2/16 levels between participants were sensitive to dietary fat and fiber intake, and future studies evaluating the 2/16-breast cancer association should control for macronutrient intake. Dithiocarbamate excretion, a biomarker of Brassica consumption, inconsistently predicted self-reported Brassica intake and is sensitive to the types of vegetables consumed. However, DTC may be useful to rank-order participants in epidemiological studies. Overall, to the extent that lower 2/16 values are associated with increased breast cancer risk, Brassica vegetables may be an important component of any strategy to reduce breast cancer risk.