Small-vessel vasculitides associated with anti-neutrophil cytoplasmic auto-antibodies (ANCA) are severe systemic diseases that may affect any organ. Increasing evidence from clinical, animal and in vitro studies indicates that ANCA are causally involved in disease pathogenesis mainly through activation of neutrophils resulting in endothelial cell injury. Recent studies suggest a previously unsuspected but crucial role for alternative pathway complement activation in ANCA disease pathogenesis. In this brief review, we will discuss the evidence for complement system activation in ANCA-associated vasculitides and propose a working model that links ANCA, neutrophils and complement activation in causing an inflammatory amplification loop that may explain the severe leukocytoclastic inflammation that is typical for ANCA-associated vasculitis.