Published data on the association between methylenetetrahydrofolate reductase (MTHFR) gene polymorphisms and breast cancer risk are inconclusive. We investigated the independent and the combined effects of two commonly occurring polymorphisms, MTHFR 677C>T (rs1801133) and MTHFR 1298A>C (rs1801131), as well as their interaction with the use of hormone replacement therapy (HRT), to determine their potential contribution to breast cancer risk. We studied 530 breast cancer cases and 270 controls of the same age and ethnicity participating in a case-control study of postmenopausal women. The duration of HRT use was ascertained through a postal questionnaire. Genotyping was conducted by TaqMan® allelic discrimination. Adjusted odds ratios and 95% confidence intervals were calculated using logistic regression. No significant association was observed between either the individual or the combined MTHFR genotypes and the risk of postmenopausal breast cancer. Additionally, no effects resulting from the interaction between MTHFR genotypes and HRT use were detected. Therefore, our data do not support the hypothesis that genetic variation in the MTHFR gene is implicated in the aetiology of postmenopausal breast cancer.