Background & objectives: High plasma homocysteine (Hcy) levels are known to be associated with coronary artery disease, but the precise level associated with an increased risk is yet controversial. Whether the beneficial effects of folic acid on arterial endothelial function persist over longer periods is not known. This study was carried out to assess whether folic acid supplementation could produce improvements in Hey levels and arterial endothelial function in the patients with unstable angina (UA) and hyperhomocysteinaemia. Methods: The plasma Hey levels of 52 cases with UA and 30 control subjects were measured by using high-performance liquid chromatography (HPLC) with fluorescence detection, plasma folic acid and vitamin [B.sub.12] levels were also measured. The patients with hyperhomocysteinaemia were treated with 5 mg of folic acid for 8 wk, and then rechecked the plasma levels of Hey, folic acid and vitamin [B.sub.12] at the end of 4th and 8th wk. Arterial endothelial function was measured as flow-mediated dilation of the brachial artery using high-resolution B-mode ultrasound in 22 cases with UA and hyperhomocysteinaemia before and after folic acid treatment.