AbstractCampylobacter‐like organisms (CLOS) occur frequently in the antrum of patients with duodenal ulcer, but their pathogenetic role has remained uncertain. This study examined prospectively endoscopic biopsies of the antrum of 109 patients with duodenal ulcer, taken before and after 4–12 weeks of treatment with either a prostaglandin E1or its placebo, for the density of campylobacter‐like organisms and the severity of antral gastritis. Antral biopsies from 30 non‐ulcer dyspeptic patients were used as controls. Active chronic antral gastritis and CLOS respectively occurred in 98% and 99% of patients with duodenal ulcer, and in 50% and 57% of the controls (P<0.005). CLOS occurred in 97% of all subjects with active chronic gastritis and in 14% of those with normal gastric mucosa (P<0.0005). The density of CLOS correlated with the severity of gastritis. After treatment with prostaglandin E1and placebo, 89% and 51% respectively of ulcers healed, and 42% and 6% respectively of gastritis improved from a moderate to a mild grade, but bacteria remained positive and persisted in the same density as before treatment. The conclusion was that CLOS, antral gastritis and duodenal ulcer are closely associated, but that healing of duodenal ulcer and improvement of antral gastritis are unaffected