Moderate hyperhomocysteinaemia (MHH) is a risk factor for arteriosclerosis and thrombosis. About 10%-20% of the normal population have homocysteine levels contributing to an increased risk for arterial and venous disease. Main regulating enzymes of homocysteine metabolism are cystathionine beta-synthase (CBS) and methylenetetrahydrofolate reductase (MTHFR). Heterozygosity for CBS deficiency is most likely not an important cause for MHH in vascular disease. A recently discovered cause of MHH is reduced MTHFR activity due to a homozygous C677T mutation in the coding region of MTHFR. This mutation has been related to an increased risk for cardiovascular disease, although a number of studies are not confirmative. The elevated homocysteine levels due to this mutation can be normalized by administration of vitamins involved in homocysteine metabolism, in particular folate.