A rapidly expanding body of data indicates that vascular endothelial cells can display a variety of proinflammatory and prothrombotic functions. This realization has led to the emergence of a new view of the vascular lining, which emphasizes the dynamic role of endothelial cells in physiologic and pathophys iologic processes. Moreover, it has resulted in an appreciation that the functional status of the endothe lium is as important as its anatomic integrity. 1 Recent in vitro studies from our laboratory have focused on two specific aspects of endothelial cell pathobiology: leukocyte-vessel wall adhesion and ini tiation of coagulation. 2-10 In particular, we have ex amined the ability of inflammatory or immune cyto kines, such as interleukin 1 (IL-1), tumor necrosis factor (TNF), and lymphotoxin (LT), to increase endothelial cell adhesiveness for blood leukocytes and to modulate its procoagulant and fibrinolytic activities. At present, we are attempting to characterize further the molecu lar structures involved in these functions, and to under stand better the regulation of their expression. It is our hope that these studies will enhance the basic under standing of the role of endothelial cells in inflamma tory and thrombotic processes and potentially provide insights for new therapeutic interventions in a variety of disease states. In this article we briefly review the in vitro studies from our laboratory and from collabora tors and relate our working hypotheses concerning the role of vascular endothelium in leukocyte adhesion and thrombosis in vivo.