['Although a number of patient and bacterial factors have been identified as predictors of treatment failure in Helicobacter pylori-associated gastritis, the causes of lack of response to treatment have not been fully elucidated. We hypothesized that bacterial infiltration of the gastric mucosa might be one of the factors responsible for treatment failure in patients harbouring the bacterium.', 'We studied 182 patients with gastritis who underwent anti-H. pylori treatment with different drugs. Gastric biopsies obtained at endoscopy, were examined for electron microscopic features of infiltration and damage. Patients were assigned to different treatment groups, and endoscopy, evaluation of H. pylori status and electron microscopy were repeated at least 4 weeks after the end of treatment.', 'The overall H. pylori eradication rate was 65%. Eradication was achieved more frequently in patients without electron microscopic features of infiltration (85%), than in those patients with the bacteria deeply embedded into the gastric mucosa (45%; P < 0.0001). No treatment appeared to be clearly superior for patients with the highest degree of mucosal infiltration.'] Does gastric mucosal infiltration by Helicobacter pylori favour bacterial survival after treatment?