Antineutrophil cytoplasmic antibodies, initially detected in the sera of patients with Wegener's granulomatosis and other forms of systemic vasculitides, have also been observed in patients with inflammatory bowel disease, with a higher prevalence in ulcerative colitis. In this study, we investigated the prevalence of these antibodies in the sera of 42 patients with ulcerative colitis and 48 patients with Crohn's disease, and the possible correlations with disease activity and extent, extraintestinal complications, and therapy. Antineutrophil cytoplasmic antibodies were found in 30 out of the 42 patients with ulcerative colitis (71.4%); only 16 out of the 48 patients (33.3%) with Crohn's disease were positive (p < 0.001), and the prevalent pattern was perinuclear. No correlations with disease activity and extent, extraintestinal complications, or surgical or medical treatment were found. Our data indicate that in the case of inflammatory bowel disease, the search for antineutrophil cytoplasmic antibodies still remains a research procedure and cannot be used for clinical diagnosis.