Autoantibodies against neutrophil cytoplasmic antigens (ANCA) produce two major immunofluorescence (IF) patterns on ethanol-fixed granulocytes: the "classical" (centrally accentuated) C-ANCA, associated with Wegener's granulomatosis (WG), and P-ANCA (perinuclear), which mainly occur in renal vasculitis. Rheumatic manifestations are an important clinical finding in systemic vasculitis, often preceding a fulminant course and sometimes imitating various rheumatic disorders. We analyzed the incidence of ANCA in rheumatic patients and looked for the frequency of rheumatic symptoms in systemic vasculitis. In WG (n = 186), we found rheumatic symptoms in 55% (myalgia, 45%; arthritis, 21%); in 90%, rheumatic complaints were associated with active vasculitis. In 730 patients with various rheumatic conditions (eg, 268 rheumatoid arthritis, 130 systemic lupus erythematosis [SLE], 32 sharp-S, 50 ankylosing spondylitis, 43 systemic sclerosis) no C-ANCA were found. On the contrary, the P-ANCA pattern was seen in seven of 62 giant cell arteritis, five of 27 Felty's/Still's syndrome, and four of 130 SLE patients in addition to renal vasculitis (21/74). We demonstrated that 95% of C-ANCA-positive sera react with proteinase 3 (PR3 or myeloblastin). Using monoclonal antibodies, we showed that PR3 is expressed on the plasma membrane of neutrophil granulocytes and monocytes; thus, PR3 autoantigens are accessible for circulating antibodies. The detection of ANCA in sera from vasculitis and other rheumatic diseases is of immunodiagnostic value and provides new insight in the pathogenesis of systemic vasculitides.