The critical role played by the vascular endothelium in the development of vasculitic disease will be discussed within this article, attention being focused on the pathogenesis of the primary vasculitides, particularly Wegener's granulomatosis, microscopic polyarteritis and Kawasaki disease. Under normal circumstances the endothelium constitutes a barrier to the efflux of plasma proteins into the extracellular tissues, serves to maintain an anti-coagulant environment and forms a platform upon which many important biological processes take place. Alteration of the balance of these functions by inflammatory mediators and cytokines leads to the formation of a pro-coagulant environment, fluid leakage and increased adhesivity of the endothelium for neutrophils and lymphocytes. Both neutrophils and lymphocytes have the capacity to damage the endothelium directly, by the secretion of oxygen radicals, enzymes and other cytotoxic molecules, or indirectly by the secretion of cytokines which may alter the biological properties of endothelial cells, a process called endothelial activation. The role played by antibodies to the endothelium and to neutrophil cytoplasm components in mediating vascular injury in systemic vasculitis is also discussed and, finally, injury precipitated by components of the coagulation/thrombotic system.