ANCA are autoantibodies directed against antigens found in cytoplasmic granules of neutrophils and monocytes. ANCA are classified according to the immunfluorescence patterns they produce on normal neutrophils and according to their target antigens. At least two fluorescence patterns can be differentiated: a granular fluorescence which is distributed regularly over the entire cytoplasm of the granulocytes, leaving the cell nuclei free (cANCA: cytoplasmic pattern), and a predominantly smooth, partly fine granular fluorescence wrapped ribbon-like around the cell nuclei of the granulocytes (pANCA: perinuclear pattern). ANCA are typically found in granulomatosis with polyangiitis (Wegener’s) (GPA), microscopic polyangiitis (MPA) including renal limited vasculitis, and Churg-Strauss syndrome (CSS), which are all forms of small-vessel vasculitis. These three diseases are grouped together as ANCA-associated vasculitides (AAV). The most important clinical symptoms of ANCA-associated vasculitides are caused by poor blood supply to organs or formation of aneurysms and bleeding due to destruction of blood vessels. Classical cANCA are present in most patients with GPA and in about 30% of patients with MPA. Classical cANCA is almost always directed against proteinase 3 (PR3), and very rarely against myeloperoxidase (MPO) or against both PR3 and MPO simultaneously. In patients with MPA and CSS, myeloperoxidase (MPO) is the main target antigen of pANCA.