Organochlorine industrial compounds, combustion products and pesticides have been widely identified in the environment and residues have been detected in extracts prepared from fish, wildlife, human tissues as well as human milk and serum. Many of these compounds possess sex steroid activities and therefore have the potential to disrupt endocrine-regulated homeostasis. Organochlorines which exhibit hormonal activity include: (i) polychlorinated biphenyls (PCBs), hydroxylated PCBs, o,p'-DDT, and other organochlorine insecticides which exhibit estrogen receptor (ER) agonist activities; (ii) p,p'-DDE, a ligand for the androgen receptor which exhibits antiandrogen activity; (iii) PCBs, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and related aromatic hydrocarbons which bind the aryl hydrocarbon (Ah) receptor and exhibit tissue-specific antiestrogenic activity; and (iv) hydroxylated aromatics which bind transthyretin, a thyroid hormone binding protein. Although, it has been suggested that the estrogenic activity of PCBs and DDE may be a contributing factor for development of breast cancer in women, levels of these compounds are not consistently elevated in breast cancer patients and there is no evidence that women occupationally-exposed to relatively high levels of PCBs or DDE exhibit an increased incidence of breast cancer. In contrast, epidemiology studies suggest that women exposed to high levels of TCDD during an industrial accident in Seveso, Italy, have a decreased incidence of both breast and endometrial cancer. Based on the dietary intake of hormone or antihormone mimics derived from natural compounds in food, the estrogenic contribution of organochlorine compounds is small and their role in development of breast cancer is questionable.