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Antineutrophil-cytoplasmic-autoantibodies (p-ANCA) are strongly associated with ulcerative colitis (UC) and may represent an indicator of genetic susceptibility to UC. To further examine whether p-ANCA may serve as a genetic marker of UC we evaluated the frequency of p-ANCA in unaffected family members of UC from a defined geographic area. A total of 110 patients with UC and 90 unaffected family members (first- or second-degree relatives) were tested. Controls included: 58 Crohn's disease (CD) patients with 25 unaffected relatives and 52 irritable bowel syndrome (IBS) patients with 20 healthy family members. p-ANCA were detected by enzyme-linked immunoassay followed by immunofluorescence. p-ANCA were detected in 57 UC patients (51.8%). Six of 90 (6.6%) unaffected relatives were positive for p-ANCA and 4 of these were from two families. In 3 of 35 families the proband and at least one unaffected relative were p-ANCA-positive. In five families with more than one member affected by UC, p-ANCA were detected in 2 of 19 (10.5%) unaffected relatives. Six CD patients (10.3%) and one (1.9%) in the IBS group were positive for p-ANCA. One family member was positive in the CD family group and 1 was positive in the control family group. In the group of families recruited for this study, p-ANCA were not more frequent in unaffected relatives of UC patients than in controls, suggesting that at least in the geographic area considered for this study p-ANCA may not represent a definite subclinical marker of susceptibility for UC.