Recently, 2 genome-wide association studies demonstrated that single nucleotide polymorphisms (SNPs) of the fibroblast growth factor receptor 2 (FGFR2) gene at intron 2 are significantly associated with the risk of female breast cancer. As the next step, it is necessary to evaluate the interaction between these SNPs and known risk factors of breast cancer because such an evaluation could elucidate mechanisms of carcinogenesis and lead to preventive advances. We conducted a case-control study of 456 newly and histologically diagnosed breast cancer cases and 912 age- and menopausal status-matched noncancer controls. The impact of 5 FGFR2 intronic SNPs on the risk of breast cancer and the interactions between these SNPs and various known risk factors of breast cancer were evaluated in both pre and postmenopausal women. We observed a statistically significant association between 4 SNPs and breast cancer risk and these 4 SNPs were in strong linkage disequilibrium in the Japanese population. rs2420946 was associated with a population-attributable risk of 17.7%. We found that FGFR2 polymorphisms interact with family history of breast cancer (interaction p = 0.003) and reproductive risk factors, namely, age at menarche (interaction p = 0.019) and parity (interaction p = 0.026). Of note, a significant association between body mass index (BMI) > or = 25 and FGFR2 polymorphism was observed among postmenopausal women (trend p = 0.012), but not among premenopausal women. In contrast, BMI < 25 had no significant association with this polymorphism regardless of menopausal status. These findings suggest that FGFR2 intronic SNPs affect the reproductive hormone-related pathway and contribute to the development of female breast cancer in the Japanese population.