Vascular diseases are commonly associated with traditional risk factors, but in the last decade scientific evidence has suggested that elevated plasma levels of homocysteine are associated with an increased risk of atherosclerosis and cardiovascular ischaemic events. Cardio- and cerebrovascular diseases are multifactorial, as their aetiopathogenesis is determined by genetic and environmental factors and by gene-gene and gene-environment interactions. Experimental studies have shown that many possible mechanisms are implicated in the pro-atherogenic effect of homocysteine. Hyperhomocysteinaemia may confer a mild risk alone, but it increases the risk of disease in association with other factors promoting vascular lesions. Variants in genes encoding enzymes involved in homocysteine metabolism, or depletion of important cofactors or substrates for those enzymes, including folate, vitamin B12 and vitamin B6, may result in elevated plasma homocysteine levels. Several studies have been performed to elucidate the genetic determinant of hyperhomocysteinaemia in patients with vascular disease, and the MTHFR 677C>T polymorphism is the one most extensively investigated. However, the lack of homogeneity in the data and the high number of factors influencing plasma homocysteine concentrations remain conflicting. Moreover, studies on the evaluation of therapeutic interventions in improving the atherogenic profile, lowering plasma homocysteine levels, and preventing vascular events, have shown inconsistent results, which are reviewed in this paper. More prospective, double-blind, randomized studies, including folate and vitamin B interventions, and genotyping for polymorphisms in genes involved in homocysteine metabolism, might better define the relationship between mild hyperhomocysteinaemia and vascular damage.