Purpose: Helicobacter pylori infection is one of the most common gastrointestinal infections worldwide; it almost invariably causes chronic gastritis. Pediatric studies may provide important insights into the mucosal immune response of H. pylori-infection, as children are not submitted to environmental factors such as alcohol, tobacco and anti-inflammatory medication. The aim of the present study was to investigate the mucosal immune response against H. pylori in clinically well-defined groups: H. pylori-positive (divided into peptic ulcer disease and gastritis) and H. pylori-negative control. Methods: Antral biopsies were obtained from 45 children undergoing an upper GI endoscopy for dyspeptic symptoms. T cells (CD3+, CD4+, CD8+) and B cells (CD20+) were analyzed by quantitative immunohistochemistry. The correlation of lymphocyte subsets of gastric mucosa with histology was evaluated. Results: T cells (CD3+, CD4+, CD8+) and B cells (CD20+) were significantly increased in the lamina propria of H. pylori-positive group (p<0.01). CD8+ T cells were significantly increased in the lamina propria of the H. pylori-positive peptic ulcer disease (p<0.01). Within the epithelium, only CD4+ T cells were significantly increased in the H. pylori-positive group (p<0.01). Gastric histological parameters had a closer correlation with lymphocytes in the lamina propria than intraepithelial lymphocytes. Conclusion: This study suggests that both T cells and B cells in the lamina propria play important roles in the local immune response of H. pylori-infected children. Furthermore, it remains to be elucidated whether CD8+ T cells in the lamina propria may contribute to peptic ulcer formation in H. pylori-infected children.