The patient was a 12-year-old girl with no comorbidities and no oral medications. She fainted after hematemesis and hematochezia and was rushed to the ER. She received a blood transfusion for severe anemia and underwent gastrointestinal endoscopy, which detected a hemorrhagic duodenal ulcer. She was treated with a proton pump inhibitor (PPI) and her symptoms improved. One month later, she presented with vomiting and poor weight gain. She underwent gastroscopy, which revealed duodenal stenosis and multiple duodenal ulcers. Duodenography showed duodenal stenosis and tasche formation. Even a narrow endoscope (7.7 mm) could not pass through the stenosis, and a biopsy of the duodenal stricture was impossible. She was managed via total parenteral nutrition and referred to our hospital. Edematous duodenal wall thickening and air-fluid levels on the right side of the duodenum were observed via abdominal computed tomography (CT), which suggested the presence of duodenal perforation or penetration. The wall of the common bile duct was thickened with enhancement. There was no swelling or mass in the pancreas. She had high serum levels of IgG4 (214 mg/dl) but no other tumor marker level was remarkable (CEA: 0.7 ng/ml, CA19-9: 7 U/ml). Serum helicobacter pylori antibody was negative. Serum gastrin was normal (87 pg/ml). C-reactive protein (0.19 mg/dl), total bilirubin (0.3 mg/dl), and amylase (115 U/l) were not elevated. Two months of PPI treatment did not improve the obstruction and multiple duodenal ulcers. CT revealed suspected duodenal perforation or penetration. Hyper-inflammation of the duodenum could lead to the Vater’s papillary deformity and, eventually, obstructive jaundice and liver damage. A pathological diagnosis was not obtained preoperatively, thus malignancy could not be ruled out. Moreover, she was unable to feed for long and, consequently, experienced poor weight gain. Therefore, she underwent pancreaticoduodenectomy. Gross examinations of the resected specimen revealed perforation and an ulcerated duodenal wall. Microscopically, a duodenal ulcer was observed, which was associated with the underlying presence of extensive sclerosing fibrosis, and the presence of lymphoplasmacytic inflammation which involved the duodenal wall. There was no malignancy. Immunohistochemical staining revealed numerous IgG4-positive plasma cells at 50/HPF, and the IgG4/IgG ratio was > 40%. There were no IgG4-positive cells in the bile duct or pancreas. Following an uneventful postoperative course, the patient was discharged on the postoperative day 16 tolerating oral intake. The serum IgG4 level decreased to 117 mg/dl postoperatively. She is currently doing well one month after surgery.