This hypothesis proposes that, in the absence of actively metastasising neoplasia, damage to the endothelium can be monitored by measuring circulating levels of von Willebrand factor. This specific product of the endothelial cell is important in thrombo-genesis as its functions include platelet aggregation and mediation of platelet adhesion to the subendothelium. High levels are found in all major risk factors of atherosclerosis, in frank atherosclerotic vascular disease and in most of the inflammatory vasculitides, and the highest levels are also associated with more severe disease and risk of mortality. It follows that high levels of von Willebrand factor may be a new risk factor for the development of thrombosis and atherosclerosis.