Antral gastritis has been described in up to 100% of patients with duodenal ulcer disease, and recently has been associated with Campylobacter pylori infection. The present study examines the effect of two ulcer therapies (cimetidine and colloidal bismuth subcitrate) on this lesion. Sixty-four consecutive patients with endoscopically diagnosed duodenal ulcer were randomized to treatment with either agent upon entry into the study. Antral biopsies were taken at both initial and final endoscopies for culture of C. pylori and assessment of degree of gastritis (as based on inflammatory cell infiltrate: grades 0 to 3). The organism was detected in 90% of the patients at initial biopsy, and grades 2 and 3 gastritis were documented in 76%. The presence of the organism and the gastritis were significantly correlated (p less than 0.001), and the bacterium was not cultured from histologically normal antral mucosa. Active chronic gastritis (grade 3) was seen in 65% of those with C. pylori, and was not found in any patient without the bacterium. Cimetidine had no effect on either the organism or the gastritis, whereas the bismuth preparation significantly (p less than 0.002) reduced the prevalence of both. The present study documents that therapy directed against C. pylori has a significant effect on the gastritis of duodenal ulcer disease and possible mechanisms for the observed effects are discussed.