We studied the prevalence of perinuclear antineutrophil cytoplasmic antibody (p-ANCA), as detected by immunofluorescence, in 290 Italian subjects. One hundred and two were affected by ulcerative colitis, 48 by Crohn's disease, 40 by gluten-sensitive enteropathy and 100 were normal subjects. The prevalence of p-ANCA was significantly higher in ulcerative colitis patients (45.1%) as compared to Crohn's disease patients (4.8%), gluten-sensitive enteropathy (0%) and normal subjects (1%; p < 0.0001 ulcerative colitis vs. all other groups). In this setting, the overall specificity of the test was 98.1% with a sensitivity of 45.1%. The specificity slightly decreased to 95.1% when ulcerative colitis patients were compared to patients with Crohn's colitis. In our series, p-ANCA appeared to be more prevalent in ulcerative colitis patients with more aggressive disease. ELISA experiments performed in order to identify the putative antigen(s) recognized by p-ANCA-positive sera showed that 8 of 12 sera positive at immunofluorescence reacted with at least one of the neutrophil preparations tested. The reactivities were directed towards various neutrophil preparations. Preabsorption with the specific antigen recognized by ELISA significantly inhibited the p-ANCA immunofluorescence reactivity indicating that p-ANCA reactivity might derive from the recognition of heterogeneous neutrophil-associated antigens.