Abstract There is increasing evidence that a raised blood level of homocysteine (HC) is a risk factor for premature atherosclerosis. With a gene frequency between one in 70 and one in 200 this condition may be more common than previously thought. It should be suspected especially in young patients in whom other risk factors are absent. The diagnosis may be made by demonstrating raised plasma HC levels, either basally or after methionine loading. Studies have shown significantly increased levels of HC in patients with premature coronary artery, peripheral vascular and cerebrovascular disease. The mechanisms by which HC produces vascular damage are, as yet, not completely understood but endothelial injury is probably a central factor. The principle of treatment is to lower HC levels in the blood by administration of vitamin B6, vitamin B12, folate or betaine. How effective this strategy will be in preventing complications is not yet known.