Polyarteritis nodosa (PAN) is the systemic necrotizing vasculitis on small and median muscular vessels. Recent studies reveal that PAN consists of classical PAN and microscopic polyangitis (MPA). MPO-ANCA, which is one of anti-neutrophil cytoplasmic antibody (ANCA), is involved in the pathogenesis of MPA. In future, by the progress of research of ANCA, the definition or criteria of systemic vasculitis including PAN or MPA will be changed.