Abstract Background: Increasing evidence indicates that genetic factors are involved in the process of carcinogenesis. BRCA1 mutation has been proven to be responsible for increased risk of ovarian cancer. However, the importance of other genetic disorders, such as MTHFR polymorphism for increased risk of carcinogenesis, is still to be determined. Abnormal methylation seems to play a significant role in ovarian cancer pathogenesis. MTHFR catalyses the conversion of 5,10- methylenetetrahydrofolate into 5- methyltetrahydrofolate which is a co-substrate in homocysteine remethylation into methionine. Thermolabile MTHFR protein variants with lower enzymatic activity are the effects of the mutation. Aim: The evaluation of 677C>T MTHFR polymorphism frequency in the group of ovarian cancer women with BRCA1 mutation. The assessment of the MTHFR 677C>T polymorphism influence on ovarian cancer risk. Methods: A group of 153 patients with ovarian cancer (Caucasian, Polish population in Wielkopolska region) were included into the study. 3 mutations: 5382insC, C61G and 4153delA in BRCA1 gene, and genotype frequency within 677C>T MTHFR polymorphism, were identified. The analysis of genotype frequency was performed by means of PCR/RFLP method. Results: 127 women without BRCA1 mutation and 26 with one of the mutations: 5382insC, C61G or 4153delA were qualified for the investigation. In the group with BRCA1 mutation, 3 out of 26 patients were with TT genotype mutation of 677C>T polymorphism (12%). Heterozygotic genotype CT appeared in 13 cases out of 26 (50%), homozygotic CC in 10 out of 26 (38%). In the group of 127 ovarian cancer patients without BRCA1 mutation, TT genotype in 677C>T polymorphism was present in 5 women (4%). Heterozygotic genotype CT appeared in 61 cases (48%), similarly to homozygotic genotype CC - 61 (48%). The highest value (OR=3.18) was obtained when comparing the homozygotic TT genotype groups. None of the obtained values was statistically significant. Conclusion: Contrary to numerous suggestions in various publications, we did not confirm the correlation between MTHFR 677C>T polymorphism and the influence on the risk of ovarian cancer in BRCA1 mutation carriers in the investigated group of Polish women.