Twenty-one patients with endoscopically confirmed duodenal ulceration, who had failed to heal with an H2-antagonist, were given omeprazole 20 mg o.m. for four weeks. Four antral biopsies from each patient were taken at endoscopy before, at the end of, and four weeks after treatment. Rapid urease test, culture, histopathology and transmission electron microscopy were carried out on these biopsies to determine the presence of Helicobacter pylori and improvement of gastritis. After four weeks of treatment, duodenal ulceration was healed in 16 (79%) of the patients; H. pylori was not detected by culture in 11 (50%) of the patients, and the associated gastritis improved in 12 (54%) patients. Four weeks after cessation of treatment the organism was cultured from antral biopsies of 18 (86%) of the patients and all of these had gastritis. Omeprazole treatment healed duodenal ulceration, improved gastritis temporarily, and suppressed but did not eradicate H. pylori.