The mechanisms underlying glomerular capillary wall injury in Wegener's granulomatosis (WG) are not well understood. Anti-neutrophil cytoplasmic antibodies (ANCA), present in sera from patients with WG, are known to stimulate respiratory burst and degranulation of primed polymorphonuclear neutrophils (PMN) in vitro. Experimental studies have shown that oxygen radical production and lysosomal enzymes are important mediators of glomerular capillary wall injury. In the present study we investigated the presence of activated PMN and the extracellular localization of lysosomal enzymes in 28 consecutive renal biopsies from patients with WG. The presence of activated PMN within the renal biopsies was compared with the capacity of ANCA, isolated from simultaneously drawn serum samples, to activate primed PMN obtained from a normal donor. Both parameters were also related to renal function. Renal biopsies were obtained from newly diagnosed WG patients before therapy had started. Activation of PMN in the biopsies was assessed by measuring hydrogen peroxide production in situ. The number of activated PMN in the biopsy correlated with the extent of impairment of renal function. Proteinase 3, myeloperoxidase, and elastase, all targets of ANCA, were localized extracellularly in renal tissue and were also found within tubular epithelial cells. All ANCA positive samples were capable of activating primed PMN. The amount of activation correlated with the ANCA titer in those samples. No correlation, however, was found between the in vitro capacity of ANCA-positive IgG fractions to activate primed PMN and the number of activated PMN present in the renal biopsy. We conclude that activated PMN producing toxic oxygen metabolites and releasing lysosomal enzymes, are present in renal biopsies from patients with WG. The amount of activated PMN present within the kidney, and not the capacity of the corresponding ANCA to activate PMN, correlates with renal tissue damage as assessed by serum creatinine levels.