Lifetime exposure to endogenous steroidal estrogens is an established risk factor for breast cancer, and exposures to other estrogenic and antiestrogenic compounds might also modify the risk of breast cancer. It has been hypothesized that synthetic estrogenic industrial pollutants such as organochlorine compounds and plant-derived estrogenic compounds also modify breast cancer risks; however, recent studies show that levels of organochlorine pollutants are similar in breast cancer patients and controls. There is evidence that synthetic and plant-derived estrogens are selective estrogen receptor modulators, which implies that these compounds can induce tissue-specific, time- and dose-dependent estrogenic or antiestrogenic responses. Therefore, the effects of synthetic or plant-derived estrogens on the incidence of breast cancer depend on both the levels and the timing of exposure to these compounds, particularly during stages of mammary gland development that are extremely sensitive to hormone levels.