Common polymorphisms in the methylenetetrahydrofolate reductase gene (MTHFR) may play a role in fetal viability. Of these polymorphisms, c.677C>T has the greatest effect, resulting in the replacement of an alanine amino acid by a valine (p.Ala222Val). In our country, our own results suggest that the homozygous prevalence of the c.677C>T mutation is 12%. Functional effects are manifested in the thermolabile and reduced functional activity of the enzyme, which reduces homocysteine-methionine conversion. The so-called hyperhomocysteinemia affects endothelial cell function and promotes thrombus formation. Determination of plasma homocysteine levels and MTHFR genotyping (most importantly determination of the c.677C>T mutation) are complementary tests, because in cases of mild hyperhomocysteinemia caused by heterozygous and especially homozygous forms of the MTHFR polymorphism, folic acid and vitamin B6, B12 supplementation is recommended, since adequate folic acid levels neutralize the potential adverse effects of the mutation. Thus, the association between the two adjacent components of MTHFR polymorphism - elevated homocysteine levels - phenotypic effect (mainly arterial, less so venous thrombosis and habitual miscarriage) is clearly demonstrated, but not between the two endpoints. This is precisely because of the possibility of externally influencing the homocysteine level and thus preventing adverse effects.