CYP17 gene is involved in steroidogenesis and steroid metabolism. Epidemiologic results on the association between the CYP17 polymorphism and breast cancer risk have been inconsistent. We examined the association between the MspAI polymorphism at +27 relative to the start of transcription in the 5'-untranslated region of CYP17 gene and breast cancer risk in Korean women. Four hundred and sixty-two incident cases and 337 controls were recruited from three teaching hospitals in Seoul during 1994-2001. Polymorphism of the CYP17 gene was determined by a single base extension assay. Demographic and lifestyle characteristics were identified using structured questionnaire. Age-adjusted (aOR) and multivariate odds ratios (& mgr;OR) and 95% confidence intervals (CI) were estimated by unconditional logistic regression. The proportions of A1/A1, A1/A2 and A2/A2 genotypes among controls were 20.8%, 45.1% and 34.1%, respectively. Compared to the A1/A1 genotype, A1/A2 or A2/A2 genotype was not statistically significantly associated with overall breast cancer risk (i.e., mOR = 1.01, 95% CI = 0.69-1.47 and mOR = 0.76, 95% CI = 0.51-1.14, respectively). However, a significant association between CYP17 A2/A2 genotype and breast cancer was observed among women aged 50 years or less (mOR = 0.58, 95% CI = 0.34-0.99, P =0.04) and leaner women (body mass index < 22 kg/ m2) (mOR = 0.48, 95% CI = 0.23-0.97, P = 0.04). Our results suggest that genetic polymorphism in 5'-untranslated region of CYP17 might play a role in breast cancer development in Korean women among younger women aged less than 50 or leaner women with body mass index less than 22 kg/m2.