To the extent that diet is involved in the etiology of breast cancer, its effect may be mediated, in part, through hormonal mechanisms. It has been suggested that the consumption of phytoestrogens is related inversely to breast cancer risk. Phytoestrogens are weak estrogens of plant derivation that may have antiestrogenic effects through competitively binding to estrogen receptors, thus diminishing the binding of stronger endogenous estrogens. This paper advances the hypothesis that, through this mechanism, dietary phytoestrogens may attenuate the adverse consequences of obesity on the development of postmenopausal breast cancer. Such an association might partly explain the low breast cancer rates observed among postmenopausal Hispanic women despite their greater adiposity, an important breast cancer risk factor. This hypothesis would lead us to expect that obesity increases the risk of postmenopausal breast cancer in women consuming small quantities of phytoestrogens but does not increase risk in women consuming larger quantities. If the hypothesis is confirmed, such as association could have important implications for reducing breast cancer risk through diet, using naturally occurring substances, particularly in women for whom postmenopausal obesity is an important health concern.