To determine the prevalence and distribution of gastric lymphoid follicles in patients with Helicobacter pylori infection, to evaluate their relationship with gastroduodenal pathology, and to assess their evolution after eradication of H pylori, mapped gastric biopsy specimens were obtained from 20 H pylori-negative normal volunteers, 25 asymptomatic volunteers with H pylori infection and no ulcer disease, 21 duodenal ulcer patients, and 16 patients with gastric ulcer. Nine infected subjects were treated by triple therapy, and biopsy specimens were obtained at 1, 4, and 12 months posttreatment. Lymphoid follicles were counted and other histologic features were scored semiquantitatively. None of the noninfected subjects had lymphoid follicles. All subjects with H pylori had follicles, which were more numerous in the antrum than in the corpus (P < .001) and on the lesser rather than on the greater curvature (P = .003). Ulcer patients had greater numbers of follicles than asymptomatic infected volunteers (P < .05). There was no relationship between number or distribution of follicles and patients' age, sex, or other mucosal inflammatory responses (except for numbers of lymphocytes: r = .785, P < .001), intensity of H pylori infection, or intestinal metaplasia. Eradication of H pylori resulted in a slow decrease (but not in the disappearance) of lymphoid follicles in all patients. Our results indicate that the careful examination of multiple specimens will reveal lymphoid follicles in the gastric mucosa of all patients with H pylori infection. The normal stomach does not contain mucosa-associated lymphoid tissue, and this study supports the concept that H pylori may be a precursor in the development of primary gastric lymphomas.