Background: TRAIL (TNF-related apoptosis-inducing ligand) is known to induce apoptosis in many cell types, and has been proposed to have a role in atherosclerosis. TRAIL was reported to be highly expressed in macrophages in atherosclerotic plaques and its receptors were known to be expressed in endothelial cells. Increased adhesion of monocytes to ECs was reported in hyperhomocysteinemia, however the mechanism or implication regarding homocysteine-induced vasculopathy has not been fully defined yet. We evaluated the role of TRAIL in ECs–monocytes interaction in induced hyperhomocysteinemia condition. Methods: SAH formation was induced intracellularly by administering homocystine(100 關M), adenosine (50 關M), and erythro-9-(2-hydroxy-3-nonyl) adenine (5 關M) to the culture media and HUVECs were incubated for 24-48 hours. TRAIL and TRAIL receptor expression was measured by FACS and Western blots. To evaluate apoptosis of HUVEC after SAH induction, cells were labeled with DiI and mixed with no labeled monocytes for 24 hours before SAH treatment. Results: 1) SAH induction was found to increase TRAIL expression in monocytes significantly, which was reduced by apocynin, a NADPH oxidase inhibitor, suggesting those effects was mediated by oxidative stress. 2) SAH induction was found to induce TRAIL receptors (DR4 and DR5) expression and to suppress TRAIL decoy receptors (DcR1 and DcR2) in HUVECs. Moreover, Soluble TRAIL increased apoptosis of HUVEC in SAH-induced group. 3) In co-culture of monocytes and ECs in the condition of intracellular formation of SAH, adhesion of monocytes to ECs was found to be increased significantly. Interestingly, more apoptotic fraction in ECs was observed in co-culture with monocytes than in EC culture alone and apoptosis was near completely blocked by TRAIL-neutralizing antibody, suggesting interaction via TRAIL-TRAIL receptor interaction might play important roles in homocysteine-induced damage in ECs. Conclusions: These results showed that homocysteine increased endothelial cells-monocytes interaction via TRAIL-TRAIL receptor system and suggested a novel mechanism of homocysteine induced vascular damage.