Vasculitis is defined by inflammation of the blood-vessel wall and forms the pathological foundation of a diverse group of individual disease entities. Among the primary systemic vasculitic diseases, Wegener's granulomatosis and microscopic polyangiitis share several common features, including pulmonary capillaritis, pauci-immune focal crescentic necrotizing glomerulonephritis, and circulating antineutrophil cytoplasmic antibodies (ANCA) that are specific for proteinase 3 or myeloperoxidase. Because both diseases are highly associated with ANCA, they are sometimes collectively referred to as ANCA-associated vasculitis. However, the presence of granulomatous inflammation in Wegener's granulomatosis represents a potentially important distinction between these two rare diseases, and it remains unclear whether . . .