Although several reports are available on the association between CYP1A1 polymorphisms and breast cancer risk in Caucasian women, it has never been reported in Japanese women. Since breast cancer incidence and clinicopathologic features of breast cancers are different between Japanese and Caucasian women, it is conceivable that the risk factors of breast cancer might also differ. In addition, a preliminary study has shown that the frequencies of the variant allele in the CYP1A1 gene are different among ethnic groups. Therefore, in the present study, we investigated the association of CYP1A1 polymorphisms with breast cancer risk in Japanese women. The association of two CYP1A1 polymorphisms, that is, 3' noncoding region (6235(T/C)) and codon 462 (Ile/Val), with breast cancer risk was analyzed by a case-control study (195 cases and 272 controls). Variant allele 6235C carriers at the 3' noncoding region polymorphism showed a significantly (p < 0.01) reduced breast cancer risk (odds ratio 0.60; 95% CI 0.41-0.88) as compared with noncarriers, and variant allele 462Val carriers at the codon 462 polymorphism also showed a significantly (p < 0.05) reduced risk (odds ratio 0.66; 95% CI 0.45-0.96) as compared with noncarriers. The relationship between the genetic polymorphisms and clinicopathologic characteristics of breast cancers was also investigated. Variant allele 6235C carriers showed a significantly (p < 0.02) higher positivity of lymph node metastasis than noncarriers (54% versus 36%), and tumors measuring less than 2 cm were significantly (p < 0.03) more frequently observed in variant allele 462Val carriers than noncarriers (50% versus 33%). These results suggest that the CYP1A1 polymorphisms would be useful for predicting breast cancer risk as well as some tumor characteristics in Japanese women.