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ASCULITIS is inflammation of vessel walls. It has many causes, although they result in only a few histologic patterns of vascular in-flammation. Vessels of any type in any organ can be affected, a fact that results in a wide variety of signs and symptoms. These protean clinical manifesta-tions, combined with the etiologic nonspecificity of the histologic lesions, complicate the diagnosis of specific forms of vasculitis. This is problematic be-cause different vasculitides with indistinguishable clinical presentations have very different prognoses and treatments. For example, a patient with purpura, nephritis, and abdominal pain caused by Henoch– Schönlein purpura usually has a good prognosis and needs only supportive care, whereas a patient with purpura, nephritis, and abdominal pain caused by microscopic polyangiitis is likely to go on to life-threatening organ failure if not treated promptly and appropriately. In recent years there has been substantial progress in identifying the attributes of specific types of vas-culitis that allow accurate diagnosis. One approach to classifying noninfectious vasculitides categorizes them, in part, on the basis of the predominant typ