Previous studies from this laboratory showed that serum anti-neutrophil cytoplasmic antibodies, distinct from those associated with active Wegener's granulomatosis, are present in the majority of patients with ulcerative colitis. In this study, the specificity of anti-neutrophil cytoplasmic antibodies for ulcerative colitis as compared with other colitides and diarrheal illness was determined. In a blinded study, test samples of serum were screened for anti-neutrophil immunoglobulin G in a fixed neutrophil enzyme-linked immunosorbent assay. Levels of neutrophil binding by immunoglobulin G and titers of anti-neutrophil immunoglobulin G in sera from patients with ulcerative colitis and patients with ulcerative colitis post colectomy were each significantly greater than the levels and titers for normal controls and patients with a variety of other colitides and diarrheal illnesses. Levels of neutrophil binding for colonic Crohn's disease, bacterial/amoebic colitis, the irritable bowel syndrome with diarrhea, and other miscellaneous diarrheal illnesses were not significantly different than the levels for normal controls. Although the levels of binding for collagenous colitis were significantly less than the levels for ulcerative colitis, they were significantly greater than the levels for normal controls. Patterns of neutrophil binding by serum samples that were positive in the enzyme-linked immunosorbent assay were determined by indirect immunofluorescence. Perinuclear staining was the predominant pattern shown by sera from patients with ulcerative colitis. The combination of a positive value in the enzyme-linked immunosorbent assay and a perinuclear immunofluorescence pattern was 60% sensitive and 94% specific for ulcerative colitis. It is concluded that anti-neutrophil cytoplasmic antibodies in ulcerative colitis are not simply an epiphenomenon related to inflammation of the colon. Identification of the antigen(s) to which these autoantibodies are directed may facilitate understanding of the underlying immune response and may allow development of an assay that is more sensitive and specific for ulcerative colitis.