Elevated plasma homocysteine is an independent risk factor for atherosclerosis and thrombosis. The exact mechanism by which homocysteine exerts its atherothrombotic action is still unclear. Accumulating evidence suggests that hyperhomocysteinaemia leads to endothelial injury and dysfunction, mediated by free radicals generated during the oxidation of homocysteine. Homocysteine also stimulates the proliferation of vascular smooth-muscle cells and inhibits the growth of vascular endothelial cells. Elevated homocysteine levels may also promote thrombosis by increased generation of thrombin. Other possible mechanisms for homocysteine-mediated atherogenesis include: the altered methylation of DNA and altered regulatory proteins associated with cell membrane, decreased bioavailability of nitric oxide, increased elastolysis and collagen accumulation, overstimulation of N-methyl-D-aspartate receptors and excessive adhesion of monocytes and neutrophils to endothelium. Understanding the mechanisms in vivo by which hyperhomocysteinaemia is associated with vascular disease may provide new approaches to prevention and treatment of atherothrombosis.