(A light- and electronmicroscopic study). We have examined the occurrence of Helicobacter pylori (HP) in 2937 gastric antral biopsy specimens from 979 patients with upper gastrointestinal symptoms. The incidence of HP proved to be 50.5%. The endoscopic diagnoses were: gastritis (62 cases), gastric erosion (425), gastric ulcer (51), duodenitis (22), duodenal erosion (119), duodenal ulcer (122) and the HP incidence was 29, 46, 63,, 50, 66 and 73%, respectively. Microscopic findings were: chronic gastritis (442), chronic active gastritis (356) and normal mucosa (181). The prevalence of HP in these groups was 43%, 78% and 11%, respectively. The activity of gastritis was in good correlation with HP infection. We did not see epithelial damage, and found only a few instances of mucus depletion. Electronmicroscopic examination was performed in order to investigate the morphology of bacterium and its relation to the mucosal cells. We observed mild loss of microvilli on the cell surface and did not see any cell invasion by bacteria.