Background; We examined the histopathological changes of the gastroduodenal mucosa after chemotherapy against Helicobacter pylori to evaluate what happened to them, because most of patients with chronic gastritis who received the triple chemotberapy felt improvement of upper gastrointestinal symptoms in spite of its 45% clearance rate of Helicobacter pylori in our previous study. Methods; Gastric and duodenal mucosal biopsy specimens were obtained before and 2-4 weeks after the triple chemotherapy. Helicobacter pylori colonization was evaluated with urease test and histopatho]ogical examinations including hematoxy]in-eosin and Warthin-Starry silver staining. Histopathological grading of gastric and duodenal mucosa was done according to the Sydney Syste and Whitehead's criteria, respectively. Resu)ts; Decreasing pathological grades of Helicobacrer pylori density, gastric neutrophil infiltration and chronic inflammatory cell infiltration was observed in 100%, 100% and 57.6% of 26 Helicobacter pylori-cleared patients after chemotherapy, respectively. And we a]so observed the decreasing pathological grades of Helicobacter pylori density, gastric neutrophil infiltration and chronic inflammatory cell infiltration in 70.8%, 63.6% and 29.4% of 34 Helicobacter pylori-persisted patients after chemotherapy, respectively. The histopathological changes of neutrophil and chronic inflamrnatory cell infiltration after chemotherapy were correlated with the histopathological changes of Helicobacter pylori density. Chronic inflammatory cell infiltration of duodenal mucosa was decreased only in 8.6% of patients after chemotherapy. Conclusions; After chemotherapy, the histopathological grades of Helicobacter pylori density, neutrophil infiltration and chronic inflammatory cell infiltration of gastric mucosa were decreased in both Helicobacter pylori-cleared and persisted groups. (Korean J Gastroenterol 1997;29:41 - 52)