Plasma homocysteine has been identified as a risk factor for arterial disease, retinal artery and vein occlusions, and other common eye diseases. The value of treating an elevated plasma homocysteine with folic acid for preventing further vascular disease has not been proven. Although secondary prevention of coronary artery disease using this approach has been unsuccessful, trials on primary prevention of stroke and loss of cognitive function with folic acid supplementation appear to be successful. Further trial data are awaited. In patients with premature retinovascular disease, the measurement of plasma homocysteine is suggested and reduction of elevated homocysteine with folic acid for secondary prevention of retinal arterial and venous occlusion. Meanwhile, the debate on fortification of flour for primary prevention of neural tube defects, which has already taken place in North America, continues in European countries. Such fortification could have an impact on primary and secondary prevention of vascular disease.