Anti-MPO antibodies, serum MPO and cytokines were examined in 106 patients with glomerulonephritis. Twenty-three patients had anti-MPO antibodies and crescent formation; 17 had crescentic glomerulonephritis (CRGN) and the remaining 6 had focal segmental necrosis with less than 50% crescent formation in the observed glomeruli. Pauci-immune CRGN accounted for 28 of 43 CRGN cases. Anti-MPO antibody titers were significantly higher in the cellular crescent stage and their titers correlated well with the number of crescents. Clinical observation revealed that 11 of 18 patients with anti-MPO antibody-associated CRGN had a respiratory tract infection prior to the onset of overt glomerulonephritis. Serum MPO was detected in 20 of 23 patients with anti-MPO antibodies and the amounts of MPO were especially high in the cellular crescent stage and correlated with anti-MPO antibodies. TNF-alpha and IL-6 were also detected in the sera in parallel with the anti-MPO antibody titers. These data suggest that anti-MPO antibodies, TNF-alpha and IL-6, induced by the infection, may activate neutrophils and MPO itself released from neutrophils, may play an important pathogenetic role in glomerular capillary necrosis leading to CRGN.