Helicobacter pylori infection is strongly associated with and is considered a common cause of gastritis. To study the relationship between H. pylori and gastritis, we examined whether a reduction occurs in acute granulocytic and chronic mononuclear inflammation of gastric mucosa after eradication therapy. The examination is based on morphometric counting and on semiquantitative estimation of the density of the inflammatory cells in endoscopic biopsy specimens from antrum and corpus. The series consisted of 23 consecutive outpatients with H. pylori-associated gastritis who received a 2-week course of triple therapy with colloidal bismuth subcitrate, amoxicillin, and metronidazole and who underwent an endoscopic follow-up for 6 weeks, 6 months (23 patients), and 12 months (21 patients). The eradication was successful in 20 patients ('responders'), who also remained H. pylori-free for 6 months, and in 18 examined patients for 12 months but was unsuccessful in 3 patients ('non-responders'). Both acute and chronic inflammation decreased significantly in intensity in responders in the follow-up. The acute inflammation had already disappeared at a 6-week control. The reduction in chronic inflammation was slower and occurred gradually within the study. At the 12-month follow-up, the corpus mucosa was interpreted as normal in all 18 patients studied, and the antral mucosa was interpreted as normal in 10 (56%) patients. No significant reduction in intensity of gastritis was found in the 3 non-responders or the 23 matched, untreated controls. We conclude that the eradication of H. pylori results in a disappearance of both acute and chronic gastritis. This supports the view that H. pylori plays a causal role in the pathogenesis of chronic gastritis.