During treatment of acute lymphoblastic leukemia, serious gastrointestinal complications rarely occur in children. Helicobacter pylori is not known as a pathogen causing peptic ulcer in children with leukemia who are receiving chemotherapy. We describe a 14-year-old girl with acute lymphoblastic leukemia in whom, during the most aggressive part of treatment, a large, life-threatening H. pylori-associated gastric ulcer developed. H. pylori infection played a role in the origin of the ulcer, as confirmed by the clinical resolution after its specific treatment. This case points to a possible role for H. pylori as a pathogen in children with leukemia who have either evident or occult gastrointestinal bleeding.