OBJECTIVE. Primary sclerosing cholangitis (PSC) is an autoimmune cholestatic liver disease of unknown etiology. The role of antineutrophil cytoplasmic antibodies (ANCAs) in the serum of patients with PSC remains unclear. We hypothesized that ANCA may be detectable in bile, potentially providing diagnostic and prognostic information. METHODS. Serum and bile were prospectively collected during endoscopic retrograde cholangiography (ERC) in 72 patients with PSC and other non-PSC obstructive biliary diseases. ANCA measurements were performed by indirect immunofluorescence (IIF). RESULTS. Immunoglobulin G (IgG) ANCA was detected significantly more often in the bile of PSC patients (15/39; 38%) than without (2/33; 6%) (p = 0.001). IgG ANCA in bile was associated with a ten times higher risk of PSC (p = 0.005). In addition, IgG ANCA positivity in bile was associated with the presence of dominant strictures (p = 0.03), cholangiographic severity (p = 0.004), number of ERC (p = 0.01) and interventions performed (p = 0.03). However, IgG ANCA in bile did not correlate with transplantation, cholangiocarcinoma or death. No association was observed between ANCA positivity in sera and ANA and ASCA positivity in sera or bile with the above-mentioned clinical features. CONCLUSIONS. The presence of ANCA in the bile of patients with PSC is a novel finding and highly suggestive of PSC. Biliary IgG ANCA correlates with the severity of bile duct strictures and the ensuing number of ERCs and interventions. Therefore, a positive ANCA status in bile may serve as a diagnostic and prognostic marker of the disease progression and biliary complications.