Folic acid supplementation lowers total plasma homocysteine (tHcy) and improves endothelial function in individuals with coronary artery disease (CAD) and in those with additional CAD risk factors. In the present study, we assessed whether endothelial function is impaired in healthy subjects with hyperhomocysteinaemia but without other CAD risk factors and whether folic acid supplementation improves endothelial function in these subjects. Flow-mediated dilatation (FMD) of the brachial artery was performed on 26 healthy subjects, age 49 +/- 2 years (mean +/- S.E.M.), with high tHcy (15.6 +/- 1.5 micromol/l) and 16 healthy age-matched subjects with low tHcy (7.9 +/- 0.6 micromol/l; P < 0.001). Subjects with high tHcy were then randomized to receive 5 mg/day of folic acid or placebo for 8 weeks in a double-blind cross-over trial with a 4-week washout. FMD was not associated with tHcy and was not different between high and low tHcy groups (7.0 +/- 0.6% compared with 6.6 +/- 1.2%, P = 0.76). Treatment with folic acid decreased tHcy by 34% in hyperhomocysteinaemic subjects ( P = 0.02 compared with placebo), but had no effect on FMD (+ 0.5 +/- 0.6% compared with -0.7 +/- 0.5%; P = 0.17 compared with placebo). In healthy subjects with hyperhomocysteinaemia, but without additional cardiovascular risk, endothelial function is unimpaired and folic acid supplementation has no additional effect.