Homocysteine is an amino acid that is involved in several key metabolic processes, including the methylation and sulphuration pathways. Blood concentrations of homocysteine are determined by various dietary factors, including folic acid and vitamin B(12), by alteration in physiology, such as renal impairment, and by variation in the activity of enzymes in the various pathways as a result of genetic polymorphisms, some of which are commonly found in the population. Hyperhomocysteinaemia has been associated with vascular disease, although whether it is cause or effect is still a matter of debate. In normal pregnancy, homocysteine concentrations fall. Disturbance of maternal and fetal homocysteine metabolism has been associated with fetal neural tube defects, with various conditions characterized by placental vasculopathy, such as pre-eclampsia and abruption, and with recurrent pregnancy loss. Apart from folate supplementation, which has been clearly shown to halve the risk of fetal neural tube defects, no other strategies have been identified in relation to homocysteine metabolism that will reliably reduce the frequency of these other common obstetric pathologies.