Ulcerative colitis (UC) is a chronic relapsing inflammatory bowel disease with the colorectum as its major target organ. Involvement of the upper gastrointestinal tract in UC is rare and presents with nonspecific endoscopic and microscopic characteristics. Recent studies have demonstrated proteinase 3 antineutrophil cytoplasmic antibody (PR3-ANCA) to be a serological marker for differentiating UC from Crohn's disease in children and for detecting disease activity and nonresponse to steroid therapy and antitumor necrotizing factor-α agents. Herein, we report a 13-year-old female patient mainly presenting with recurrent bilious vomiting who was initially diagnosed with acute gastroenteritis. Intestinal pseudo-obstruction was confirmed through observation of a patent but segmentally dilated jejunum in the barium follow-through examination and other imaging; such obstruction can be attributed to backwash ileitis, superior mesenteric artery syndrome, ileus due to hypokalemia, or PR3-associated enteritis. Laboratory data revealed leukocytosis with neutrophil predominance and serum antinuclear antibody and PR3-ANCA positivity. Overlapping syndrome with autoimmune diseases was suspected. Pathology revealed a crypt abscess with aggregates of neutrophils consistent with UC but did not indicate vasculitis. The 