The strong association of antineutrophil cytoplasmic autoantibodies (ANCA) with certain forms of small vessel vasculitis suggests a pathogenic role of these autoantibodies in the disease process. In vitro, ANCA can activate neutrophils and monocytes to produce reactive oxygen intermediates, to release lysosomal enzymes, and to secrete proinflammatory cytokines. More recently, it was demonstrated that antimyeloperoxidase ANCA can induce systemic vasculitis and glomerulonephritis in mice. Taken together, these data provide convincing evidence that ANCA are indeed pathogenic.