The specificity of anti-neutrophil cytoplasm antibodies (ACPA/ANCA) was investigated in patients suffering from Wegener's granulomatosis (WG), various forms of systemic diseases including vasculitides (non-WG), and different types of biopsy-proven glomerulonephritides. In particular, the diagnostic significance of ACPA/ANCA was assessed in patients affected by rapidly progressive glomerulonephritis with and without systemic manifestations. From 25 patients with active Wegener's granulomatosis 22 showed the classical diffuse finely granular cytoplasmic staining of neutrophils as did 4/31 patients with idiopathic rapidly progressive glomerulonephritis. One patient with biopsy confirmed Wegener's granulomatosis, six patients with microscopic polyarteritis and 3/31 patients with idiopathic rapidly progressive glomerulonephritis showed a focal cytoplasmic staining exhibiting a rosette-like pattern. One further patient with Wegener's granulomatosis displayed a prominent fluorescence of the outer nuclear membrane as well as a punctate-diffuse nuclear staining resembling that of granulocyte specific antibodies. Another patient with active Wegener's granulomatosis did not react in the ACPA/ANCA-test. These findings demonstrate different staining patterns of neutrophils which are related to different clinical entities within the spectrum of small vessel vasculitis. Moreover, they point out that different antigens are involved in the various types of vasculitis. In classical cases of Wegener's granulomatosis, but not in other forms of vasculitis, the titer of ACPA/ANCA showed a close relationship to the number of organs involved.