Chronic infections may mimic antineutrophil cytoplasmic antibody (ANCA)-associated vasculitides (AAV). We investigated which markers may help in the diagnosis and the prognosis of infections associated with proteinase 3 (PR3) and myeloperoxidase (MPO)-ANCA. In this study (1993-2008)--with an average follow-up of 5.1 years--we compared 66 AAV patients with 17 PR3 and/or MPO-ANCA-positive patients with protracted bacterial (11/17) or viral (6/17) infections. Seven of 17 patients had subacute bacterial endocarditis (SBE), while six of 17 patients had various autoimmune manifestations of chronic hepatitis C virus (HCV) infection. We determined ANCA, antinuclear antibodies, anti-PR3, anti-MPO, anticardiolipin (aCL), antibeta 2 glycoprotein I (beta2-GP I), cryoglobulins, C3, and C4. Patients with infections were younger than AAV patients (p < 0.01). There was no difference in frequency of renal and skin lesions. AAV patients more frequently had pulmonary and nervous system manifestations (p < 0.01). Patients with infections more frequently had dual ANCA (high PR3, low MPO), aCL, anti-beta2-GP I, cryoglobulins, and hypocomplementemia (p < 0.001). Immunosuppressive therapy (IST) was used in five 17 patients who had persistently high ANCA, cryoglobulinemia, and hypocomplementemia. There was no difference in frequency of lethality and renal failure in the two study groups. In patients who are PR3- and/or MPO-ANCA positive, SBE and HCV infection should be excluded. Although similar in renal and skin manifestations in comparison to AAV, only patients with infections developed multiple serological abnormalities. In patients with infections, concomitant presence of ANCA, cryoglobulins, and hypocomplementemia was associated with severe glomerulonephritis. The serological profile should be repeated after specific antimicrobial or surgical therapy, since some cases might require IST.