Vasculitis is a clinicopathologic process characterized by inflammation and necrosis of blood vessels. Certain disorders have vasculitis as the predominant and most obvious manifestation, whereas others have various degrees of vasculitis in association with other primary disorders. Within the entire spectrum of vasculitis virtually any size or type of blood vessel in any organ system can be involved. Most of the vasculitides can be associated directly or indirectly with immunopathogenic mechanisms. In this regard, immune complex mediation is being increasingly recognized as the underlying mechanism in several of the vasculitides. With clinical, pathologic, and immunologic criteria, certain vasculitic disorders can be clearly recognized and categorized as distinct entities, whereas in others there is an overlap of different diseases within a broader category. In recent years, several of the more serious vasculitides, such as Wegener's granulomatosis and the systemic necrotizing vasculitides of the polyarteritis nodosa group, which formerly had extremely poor prognoses, have been shown to be extraordinarily responsive to chronic low-dose cytotoxic therapy, particularly cyclophosphamide.