A case-control study was conducted to investigate the association of two genetic polymorphisms (1931T/C and 1951G/A) in the promoter region of the CYP17 gene with breast cancer risk in Japanese women. No significant association was observed between CYP17 polymorphism(1951G/A) and breast cancer risk (odds ratio (OR) = 1.71, 95% confidence interval (CI): 0.28-1.84). In contrast, a significant increase in breast cancer risk (OR= 1.82. 95% CI: 1.07-3.12) was observed in CYP17(1931C/C) homozygotes compared with CYP17(1931T/C) heterozygotes and CYP17(1931T/T) homozygotes when women aged > or = 55 years were considered, but such a significant increase was not observed when women aged < or = 54 years were considered (OR = 0.96, 95% CI: 0.56-1.63). These results suggest that CYP17 polymorphism(1931T/C) would be useful in the selection of Japanese women at a high risk for developing breast cancer at the age of > or = 55 years.