The results of many clinical studies suggest homocysteine (Hcy) as a causative risk factor for atherosclerosis. However, the mechanisms by which Hcy might cause endothelial cell injury have still remained unclear. TRAIL (TNF-related apoptosis-inducing ligand) - death receptor interaction is known to induce apoptosis in many cell types, and is also reported to have a role in atherosclerosis. Here we evaluated the role of TRAIL in endothelial cells - monocytes interaction in hyperhomocysteinemia condition. S-adenosylhomocysteine (Ado-Hcy) was induced intracellularly by administering homocystine (50 關mol/L and 100 關mol/L), adenosine (50 關mol/L), and erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA, 5 關mol/L) to the culture media of human umbilical vein endothelial cells (HUVECs) and of human monocyte / macrophage THP-1 cell line. Ado-Hcy treatment induced apoptosis of HUVECs in dose-dependent manner. Interestingly, when HUVECs were co-cultured with THP1 cells, there was marked increase in apoptosis. We found that Ado-Hcy treatment induced HUVEC to express TRAIL death receptors, DR4 and DR5, while it did THP-1 cells to express TRAIL. When HUVECs were treated with soluble TRAIL in the presence of Ado-Hcy, Akt phosphorylation was found decreased leading to increased apoptosis. In the co-culture experiment with HUVECs and THP-1 cells in the presence of Ado-Hcy, apoptosis of endothelial cells also increased in the same manner as that with soluble TRAIL treatment. When TRAIL was blocked by neutralizing antibody, apoptosis induced by THP-1 cells was near completely reversed. These results suggested that Ado-Hcy increased endothelial cells - monocytes interaction via TRAIL - death receptor system, thus inducing apoptosis in endothelial cells.