The prevalence of antineutrophil cytoplasmic antibodies was evaluated in patients with ulcerative colitis, primary sclerosing cholangitis, and various other gastrointestinal and hepatobiliary diseases to define the sensitivity and specificity of the test. The presence of antineutrophil cytoplasmic antibodies was detected in alcohol-fixed cytospin preparations of peripheral blood neutrophils with an indirect immunofluorescence technique. A perinuclear staining pattern was considered positive. Thirty-six of 50 patients (72%) with ulcerative colitis and/or primary sclerosing cholangitis had positive results. Twenty-two of 210 patients (10%) in the control group had positive findings, including a significant proportion of patients with autoimmune hepatitis (50%) and non-A, non-B and non-C hepatitis (27%). This test for antineutrophil cytoplasmic antibodies has a sensitivity of 72% and specificity of 90% for either ulcerative colitis or primary sclerosing cholangitis. It may be useful in the differential diagnosis of Crohn's disease and ulcerative colitis and in the early diagnosis of ulcerative colitis. It also may be employed to distinguish primary biliary cirrhosis from primary sclerosing cholangitis.