Abstract Background A growing body of evidence suggests that variations in the levels of folate may contribute to the development of cancer. A functional polymorphic variant (C→T substitution at nucleotide 677) in the 5,10-methylenetetrahydrofolate reductase ( MTHFR ) gene results in the conversion of an alanine to a valine and may modify the risk of breast and other cancers. Method We have investigated the possible influence of this MTHFR variant on breast cancer risk in a case-control study of 233 healthy women and 335 women who had breast cancer that occurred under the age of 40 years, bilateral breast cancer or a family history of breast cancer. Results A significant excess of the valine genotypes was observed among the cases (odds ratio 1.43, 95% confidence interval 1.02–2.00). The effect was more pronounced among the cases with a breast cancer diagnosis under the age of 40 years, with an odds ratio of 1.66 (95% confidence interval 1.12–2.41). A nonsignificant excess of the valine genotypes was observed among the cases with a family history of breast cancer or bilateral breast cancer. Conclusions The low activity C677T (valine) genotype of MTHFR may increase the risk of early onset breast cancer.