Similarly as in other inborn metabolic diseases the cause of hyperhomocysteinaemia are interactions between genetically conditioned changes most frequently due to reduced cystathionine-beta synthase activities and negative factors of the external environment. Negative environmental factors include above all a high dietary animal protein consumption which is the main methionine donor and a low intake of protein of plant origin. Another negative factor is a low intake of foods of plant origin. Fruits and vegetables are among others important sources of folic acid and pyridoxine. Substitution therapy with vitamin preparations is essential in homozygotes and in high risk heterozygotes of cystathionine beta-synthase. This treatment is also necessary during the periconception period in hyperhomocysteinaemic fertile women to reduce the risk of neurotubal defects in their future children. So far investigations are lacking which would provide evidence of a reduced risk of ischaemic heart disease and other cardiovascular diseases in isolated treatment of mildly elevated levels of plasma homocysteine. To elucidate the part played by hyperhomocysteinaemia in hastening of the atherogenetic process further studies are essential, focused on the interaction of elevated homocysteine plasma levels, dyslipoproteinaemias, hyperfibrinogenaemia and other metabolic indicators in this process.