Abstract: Objective: To investigate the relationship between the CYP17 5'-untranslated region MspA1I polymorphism and the incidence of breast cancer. Methods: Risk factors and CYP17 genotyping data were obtained from a Chinese popula-tion-based case-control study including 299 cases of breast cancer and 242 normal controls. The MspA1I polymorphism was detected using the PCR-restriction fragment length polymorphism (PCR-RFLP) assay, and the genotypes identified were labeled homozygous wild type(A1A1), heterozygous variant(A1A2), or homozygous variant(A2A2). Crude and adjusted odds ratios (OR) were evaluated and 95% confidence intervals (CIs) were calculated by unconditional logistic regression analyses. Results: In breast cancer cases, the prevalence rates of CYP17 genotype A1A1, A1A2, and A2A2 were 15.7%,56.2%, and 28.1%, respectively. In the control group, the prevalence rates of CYP17 genotype A1A1, A1A2, and A2A2 were 18.2%, 51.7%, and 30.2%,respectively. The frequencies of CYP17 A1 and A2 alleles were 43.8% and 56.2% in breast cancer patients, respectively, and 44% and 56% in the control group, respectively. No significant difference was found in the genotype distributions or allele frequencies of CYP17 polymorphism between the two groups (P>0.05). CYP17 A1A2 and A2A2 genotypes were not associated with breast cancer, with odds ratios(ORs) of 0.80(95% CI 0.50-1.27) and 0.93(95% CI 0.55-1.56). Stratified analysis showed that the postmenopausal cases with allele A2 were more likely to have had their first pregnancy at a later age(OR 2.413, 95% CI 1.035-5.623). Multivariate analysis showed that early menarche (<13years), later age during the first pregnancy( ≥25years) or last pregnancy(≥30years), and history of benign breast dis-ease were the risk factors for breast cancer. Conclusion: The presence of the MspA1I polymorphism in the CYP17 gene's 5'-UTR may not be a risk factor for breast cancer. Individuals with allele A2 who have their first pregnancy at a later age (≥25years) have a higher risk of developing breast cancer.