The inflammatory response to Helicobacter pylori in adults has been well characterized. In children, studies of H. pylori gastritis have been based on small numbers of patients and have used loose definitions of the inflammatory components. Comparative study of the inflammatory response to H. pylori in children and adults can help our understanding of the natural history of this infection. Using the Updated Sydney Classification, we performed a retrospective, blinded pathology review of gastric biopsy specimens from 42 children and 40 adults with H. pylori infection, and we quantified each inflammatory cell separately (neutrophils, lymphocytes, plasma cells, and eosinophils). Differences in inflammatory cell components of children and adults were assessed by logistic regression analysis. More children had marked amounts of H. pylori (P = .05) and mild degree of neutrophils (P = .02), plasma cells (P = .005), and eosinophils (P = .0001) compared with adults. No differences existed when quantifying mononuclear cells or atrophy. Ulcers and intestinal metaplasia were present only in adults. We found that the numbers of inflammatory cells present in H. pylori-infected biopsy specimens is different in children and adults. We hypothesize that these differences demonstrate how H. pylori infection evolves from an acute childhood infection to an adult chronic disease.