OBJECTIVE We wished to demonstrate that gastric epithelial cells infiltration by HP is associated with the active inflammatory response and the severity of gastritis in the gastric antrum of patients harboring the bacterium. METHODS We studied 129 patients with HP-associated gastritis and 60 HP-negative controls with gastritis of different origin. Gastric mucosal biopsies were obtained from all subjects at endoscopy and were examined for histological features of active inflammation and type of gastritis, as well as for electronmicroscopical features of invasion and damage, according to a four-degree classification (range 0-3). RESULTS At entry, the presence of acute inflammatory activity, defined according to the presence of a polymorphonuclear cell infiltrate, was significantly greater in HP-positive patients than in controls (p < 0.00001) and was well related to the depth of mucosal invasion (p < 0.001). Accordingly, the prevalence of chronic atrophic gastritis was higher in HP-positive patients (p < 0.02 vs. controls) and at grade 3 of invasion (p < 0.04 vs. grade 1 and 2). Peptic ulcers were more frequent in grade 3 patients (p < 0.04). CONCLUSIONS Gastric epithelial cell infiltration and damage by HP, as assessed by electron microscopy, is an important feature of HP-associated gastritis due to its histological and clinical correlates.