Objectives: To determine the accuracy of anti-neutrophil cytoplasmic antibodies (ANCAs) and anti-Saccharomyces cerevisiae antibodies (ASCA) in distinguishing patients with inflammatory bowel disease from patients with other disorders, seen in a pediatric gastroenterology clinic setting, and in distinguishing ulcerative colitis (UC) from Crohn’s disease (CD). Study design: Serum samples from 120 children with new or established diagnoses of UC (n = 25) or CD (n = 20) and control children (n = 74) were analyzed in blinded fashion for the presence of IgG ANCAs and IgA and IgG ASCA. Results: The highest sensitivity for detecting inflammatory bowel disease, 71%, was achieved by using ANCAs and ASCA together. The best test for UC was ANCAs, which had a sensitivity of 80%. However, the ANCA pattern characteristic of UC, perinuclear ANCAs eliminated by DNAse, had a sensitivity of 60%. High-titer ANCAs were specific for UC, whereas ASCA were specific for CD. Conclusions: Testing for ANCAs and ASCA together did not achieve sensitivity necessary for population screening. However, ANCAs and ASCA may be helpful in evaluating children suspected of having inflammatory bowel disease and in distinguishing UC from CD. (J Pediatr 1999;134:447-52)