The spectrum of vasculitis is diverse, and numerous entities do not fit the aforementioned broad categories. Examples of these include Buerger's disease; vaso-occlusive vasculitis of the lower extremities associated with cigarette smoking; Behçet's disease, which is prevalent in the Orient and Middle East and is characterized by recurrent aphthous stomatitis, genital ulcerations, uveitis, meningoencephalitis, and phlebitis; and isolated central nervous system vasculitis, a rare disease with a poor prognosis that primarily affects intracranial arteries without a systemic acute-phase response. Improvement in the classification and definitive therapy of vasculitis awaits identification of etiologic agents and definition of host factors and the immune response responsible for the pathology.