The purpose of this protocol is to study the natural history of idiopathic systemic vasculitis in children and adults. The idiopathic vasculitides are a group of rare, systemic diseases involving inflammation of arteries and other tissue with resulting organ- and lifethreatening disease courses. The different forms of idiopathic vasculitis are typically classified based upon the predominant size of the arteries affected in each condition, including small vessel vasculitis [granulomatosis with polyangiitis (GPA, Wegener s), microscopic polyangiitis (MPA), and eosinophilic granulomatosis with polyangiitis (EGPA, Churg-Strauss)]; medium vessel vasculitis [polyarteritis nodosa (PAN)]; and large vessel vasculitis [giant cell arteritis (GCA), Takayasu s arteritis (TAK), idiopathic aortitis (IA)]1. Although patients with each type of vasculitis manifest disease-specific aspects of illness, there are substantial disease and treatment burdens common to patients with vasculitis. For each type of idiopathic vasculitis, the disease course is often chronic, relapse is common and unpredictable, organ and tissue damage can accrue over time, new symptoms can occur late into the disease course, and treatment is often associated with toxicity and serious side effects.