OBJECTIVE To elucidate whether sera from patients with rheumatoid arthritis (RA) contain antineutrophil cytoplasmic antibodies (ANCA) or granulocyte specific antinuclear antibodies (GS ANA), or both, and to analyze possible correlations with different clinical and laboratory data. METHODS Forty-seven consecutive outpatients with RA were included. Control sera were obtained from patients with well defined rheumatic diseases and from healthy individuals. Serum samples were examined by indirect immunofluorescence (IIF) on both ethanol and paraformaldehyde fixed neutrophils and by ELISA using as substrates myeloperoxidase (MPO), proteinase 3, and a purified extract of alpha-granules. ANA were detected by IIF using cultured HEp-2 cells. RESULTS Twenty-three patients (49%) had a perinuclear pattern (p-ANCA) by ethanol fixation, of which only 2 became cytoplasmic on paraformaldehyde fixed cells. These 2 patients also had a positive myeloperoxidase ELISA, while none of the remaining 45 had a positive result from the 3 ELISA performed. All 21 patients (45%) with a p-ANCA pattern that was not modified by paraformaldehyde fixation had a specific immunostaining upon examination at high power magnification; we termed this GS ANA specific pattern. The specificity of this pattern was further confirmed by a doubled blind test performed by 2 independent observers. In our study, all GS ANA pattern positive sera fulfilled the previously known definition of these antibodies. We found no relationship between GS ANA and variables such as disease duration and activity, rheumatoid factor, and vasculitis. Notably, 2 RA patients with "true" ANCA (anti-MPO antibodies) had an associated pulmonary-renal syndrome (microscopic polyangiitis). CONCLUSIONS Most p-ANCA in our series of patients with RA did not seem to correspond to "true" ANCA but to antibodies directed against nuclear or perinuclear antigenic constituents of the neutrophils (GS ANA). The observation of their distinctive and specific immunostaining pattern, when screening patients for the presence of ANCA by IIF, may alert us to the possible presence of RA.