Homocysteine, a sulphur amino acid, is a branch-point intermediate of methionine metabolism. It can be degraded in the transsulphuration pathway to cystathionine, or remethylated to methionine via the remethylation pathway. In both pathways, major genetic defects that cause enzyme deficiencies are associated with very high plasma homocysteine concentrations and excretion of homocystine into the urine. Mildly elevated plasma homocysteine concentrations are thought to be an independent and graded risk factor for both arterial occlusive disease and venous thrombosis. Genetic defects in genes encoding enzymes involved in homocysteine metabolism, or depletion of important cofactors or (co)substrates for those enzymes, including folate, vitamin B(12) and vitamin B(6), may result in elevated plasma homocysteine concentrations. Plasma homocysteine concentrations are also influenced by dietary and lifestyle factors. In the last decade, several studies have been conducted to elucidate the genetic determinants of hyperhomocysteinaemia in patients with cardiovascular disease. We report on both environmental and genetic determinants of hyperhomocysteinaemia and give a detailed overview of all the genetic determinants that have been reported to date.