Epidemiological studies have shown that higher blood homocysteine levels appear to be associated with higher risks of coronary, cerebral, and peripheral vascular disease and are inversely related to blood levels of folate and of vitamin B12 and vitamin B6. However, observational studies cannot exclude the possibility that elevated homocysteine levels may be associated with some other factor, rather than being causally related to vascular disease. Large-scale clinical trials of sufficient dose and duration of treatment are required to test this hypothesis, but there was substantial uncertainty about the optimal vitamin regimen to test in such trials. A meta-analysis of 12 randomized trials of vitamin supplements to lower homocysteine levels was carried out to determine the optimal dose of folic acid required to lower homocysteine levels and to assess whether vitamin B12 or vitamin B6 had additive effects. This meta-analysis demonstrated that reductions in blood homocysteine levels were greater at higher pretreatment blood homocysteine levels and at lower pretreatment folate concentrations. After standardization for a pretreatment homocysteine concentration of 12 micromol/L and folate concentration of 12 nmol/L (approximate average concentrations for western populations), dietary folic acid reduced homocysteine levels by 25% (95% confidence interval [CI]: 23 to 28%) with similar effects in a daily dosage range of 0.5 to 5 mg. Vitamin B12 (mean 0.5 mg) produced an additional reduction in blood homocysteine of 7%, whereas vitamin B6 (mean 16.5 mg) did not have any significant effect. Hence, in typical populations, daily supplementation with both 0.5 to 5 mg folic acid and about 0.5 mg vitamin B12 would be expected to reduce homocysteine levels by one quarter to one third (from about 12 micromol/L to about 8 to 9 micromol/L). Large-scale randomized trials of such regimens are now required to determine whether lowering homocysteine levels by folic acid and vitamin B12, with or without added vitamin B6, reduces the risk of vascular disease.