Although Fas-mediated cell death may play a role in atherogenesis, causal data in support of this hypothesis are lacking. The present study investigated the possibility that endothelial cells are involved in vascular smooth muscle cell (VSMC) apoptosis via the Fas-FasL pathway, and hence in atherogenesis. FACS analysis detected FasL on the surface of human umbilical vein endothelial cells (HUVECs) and immunofluorescence staining of the HUVECs demonstrated high levels of FasL in the intracellular compartment. FasL was down-regulated 4 h after tumor necrosis factor (TNFalpha) treatment, coinciding with maximal surface expression of the adhesion molecules vascular cell adhesion molecule-1 and E-selectin. However, the down-regulation of FasL expression was transient, as surface expression returned within 24 h of TNFalpha treatment. When cocultured with VSMCs, the FasL-expressing EC could kill the VSMCs in a manner that could be blocked by recombinant Fas-Fc, deployed as a soluble receptor for Fas. Moreover, when human coronary arteries were studied with immunohistochemistry using G247-4 monoclonal antibody for the detection of FasL, few FasL positive EC were observed in diffuse intimal thickening. In contrast, endothelium overlying the plaque showed prominent and uniform expression of FasL. These findings suggest that the Fas/FasL pathway can be used by EC to induce VSMC apoptosis in the atherosclerotic lesion.