The purpose of the study was to evaluate the long-term effects of vagotomy on the morphological status and Helicobacter pylori infestation of the gastric mucosa. Endoscopy with biopsies (2 from antrum and 2 from corpus) was performed in 317 patients on whom vagotomy had been performed on an average 8 years earlier. The success of the vagotomy was estimated by the endoscopic Congo Red technique in 270 cases. Non-operated 187 duodenal ulcer patients were examined endoscopically and bioptically in a similar way and used as a reference series. Helicobacter pylori (HP) was determined by Giemsa staining of biopsy specimens in both series. In non-operated dudoenal ulcer patients, gastritis and HP behaved as expected from the data in literature: antral gastritis was present in nearly all cases (96%), while the corpus mucosa was normal or the process was retarded at the stage of superficial gastritis (76%) and atrophic changes were virtually lacking (1%). On the basis of the Congo Red test the vagotomized patients were separated into two groups: successfully operated, i.e., complete vagotomy, and incomplete vagotomy groups. The results of the examinations were independent of the kind of vagotomy performed, but related significantly to its completeness. The incomplete and complete groups differed significantly. The prevalence of atrophic changes (29%) in the corpus was significantly higher and that of superficial gastritis (69%) lower in the complete than in the incomplete vagotomy group, in which the prevalences were 12% and 85%, respectively. Likewise the prevalence and density of HP was lower in the complete vagotomy group but the difference was not statistically significant.(ABSTRACT TRUNCATED AT 250 WORDS)