Autoantibodies to MPO are associated with various forms of systemic vasculitis, including the renal limited form described as idiopathic crescentic glomerulonephritis. In vitro the antibodies are able to further activate primed neutrophils to the production of reactive oxygen species and the release of lysosomal enzymes. In vivo experimental studies in which an autoimmune response to MPO was induced in rats have demonstrated the in vivo potential of the autoantibodies to aggravate subclinical inflammatory lesions. In the right context, vasculitis and glomerulonephritis can ensue. Further studies are being directed to the precise characterization of autoimmune responses in order to obtain clues for the etiopathogenesis of the associated diseases.