Eosinophilic gastrointestinal disorders (EGIDs) cause various gastrointestinal symptoms due to infiltration of eosinophils into the gastrointestinal tract. Helicobacter pylori (H. pylori) is a microorganism that is associated with various diseases such as autoimmune diseases. In recent years, H. pylori is considered protective in inflammatory bowel diseases and gastrointestinal autoimmune disorders but is not known to be protective in EGIDs. A 14-year-old boy presented with epigastric pain and nausea, without diarrhea. His symptoms were not associated with meals. Blood examination showed an eosinophil count of 1,666 cells/µL (17.0%) and an interleukin-5 (IL-5) level of less than 3.9 pg/mL. Esophagogastroduodenoscopy showed chronic gastritis and duodenal ulcers. Capsule endoscopy and colonoscopy showed no abnormal findings. The patient was diagnosed with chronic gastritis due to H. pylori infection and eosinophilic duodenal ulcers. H. pylori eradication was performed. However, the abdominal pain worsened with elevated peripheral eosinophil count [2,314/µL (26%)] and serum IL-5 level (8.0 pg/mL). Montelukast administration improved the symptoms and laboratory findings [peripheral eosinophil count, 330/µL (5.9%); IL-5, < 3.9 pg/mL]. EGIDs should be considered as a cause of duodenal ulcers. H. pylori may be protective in EGIDs. Montelukast monotherapy may be considered as a first line treatment for eosinophilic duodenal lesions.