Aim.To determine a potential role of single-nucleotide polymorphisms in genes involved in DNA methylation (MTHFR, MTR, TYMS) in the breast cancer risk and risk of leukemia in a case-control study from West Ukraine. Methods. Genotyping of MTHFR 677 C>T, MTR 2756 A>G and TS 3R2R, TS 3RG>C was performed in 60 patients with leukemia, 90 patients with breast cancer and in 100 persons from a control group. The molecular-genetic analysis was performed by Polymerase Chain Reaction and Restriction Fragment Length Polymorphism analysis. A statistical analysis was conducted by Chi-square tests and odds ratio (OR) calculation. Results. We did not observe any significant difference in genotype frequencies of the MTHFR and TYMS polymorphisms between the patients and the controls. The MTR 2756AA genotype frequency was significantly higher in the breast cancer patients vs. control (0.67 vs. 0.50, p = 0.02) while the difference between the leukemia patients and the control group was not statistically significant. The increased risk of breast cancer development was associated with the MTR 2756AA genotype (OR = 2.00, CI -95 %:1.11 -3.60) and the MTR 2756A allele (OR = 1.75, CI -95 %:1.08 -2.84). Conclusions. Our findings show that West Ukrainian inhabitants carrying at least one MTR 2756A allele have a significantly increased risk of breast cancer. K e y w o r d s: methylation, polymorphism, gene, leukemia, breast cancerBiomedicine ISSN 1993-6842 (on-line); ISSN 0233-7657 (print)