Background: Folate deficiency is implicated in cancer development. Single nucleotide polymorphisms in the methylenetetrahydrofolate reductase (MTHFR) gene can modulate the effect of folate. In this case-controlled study, a possible effect of the common MTHFR C677T (ala -> val) polymorphism on breast cancer susceptibility in Turkish patients was investigated. Materials and Methods: Polymorphism analysis was performed by melting curve analysis. Results: The variant allele valine (677T) was more frequent among the patients (30.1%) than in controls (23.9%). This difference was weakly significant (p=0.046; OR=1.37) and due to a significantly higher frequency of the valine homozygotes (677TT) among the patients (12.1% vs. 5.4%; p=0.013, OR=2.5). Among the patients diagnosed at more than 40 years of age, a more pronounced association of the valine homozygotes with breast cancer risk was observed (p=0.009; OR=3.3). Conclusion: Homozygosity for the low-activity C677T genotype (TT) may represent a genetic determinant increasing breast cancer risk
