Most epidemiological studies of the association between breast cancer risk and exposure to organochlorine pesticides or polychlorinated biphenyls (PCBs), which are suspected endocrine disrupters and potential risk factors for human breast cancer, have been conducted in western countries, and the majority of results have been null and the rest inconsistent. Here, we examined these associations in Japanese women in the largest study in Asian women to date. The study was a matched case-control study of breast cancer with 403 eligible matched pairs from May 2001 to September 2005 at four hospitals in Nagano Prefecture, Japan. Serum samples were measured for PCBs and nine pesticide-related organochlorines, including dichlorodiphenyltrichloroethane (DDT). Odds ratios of breast cancer or its hormone-receptor-defined subtypes according to serum organochlorines were calculated. No increase in the risk of breast cancer was seen among women with higher serum concentrations of any organochlorine: o,p'-DDT, p,p'-DDT, p,p'-dichlorodiphenyldichloroethylene, hexachlorobenzene, beta-hexachlorocyclohexane, trans-nonachlor, cis-nonachlor, oxychlordane, mirex, or PCBs. Rather, higher serum levels of cis-nonachlor, mirex, or total PCBs were associated with a decreased risk of breast cancer Overall, these results suggest that breast cancer risk in Japan, a low-incidence country, is similar to that in western countries in terms of organochlorine exposure.