Anti-neutrophil cytoplasmic antibodies (ANCA) are a family of autoantibodies that react with proteins predominantly expressed in cytoplasmic granules of polymorphonuclear neutrophil granulocytes (PMNs). ANCA was initially detected using indirect immunofluorescence, allowing for different patterns such as p-ANCA (perinuclear) and c-ANCA (cytoplasmic) to be distinguished. Today it is common to detect the antibodies by immunochemical assays such as ELISA using purified proteins as antigens. The strongest association with ANCA is found in the pauci-immune small vessel vasculitides granulomatosis with polyangiitis (GPA) and microscopic polyangiitis (MPA). There is compelling evidence that ANCA contributes to the pathogenesis in these conditions. ANCA also occurs in 30%-40% of patients with eosinophilic granulomatosis with polyangiitis (EGPA) and anti-GBM disease, but is uncommon in other forms of vasculitis. ANCA with different specificities have been described with varying frequencies in diseases such as systemic lupus erythematosus (SLE), rheumatoid arthritis, inflammatory bowel disease, endocarditis, chronic infections and hematopoietic malignancies. ANCA can also develop as an adverse event during pharmacological treatment. These entities are treated quite differently, with therapies ranging from immunosuppressive agents over antibiotics to simply removing the causative drug. A positive ANCA test thus requires a careful diagnostic work-up. (C) 2016 Elsevier B.V. All tights reserved.