In a prospective study of 70 patients with epigastric pain, gastroduodenoscopy revealed gastric and/or duodenal ulcers in a total of 41 and no ulceration in the remainder. Biopsies were taken to assess the severity of gastritis and the presence of Campylobacter pylori (CP) by histology and culture. Gastritis was found in 54 patients. CP was detected in 78% of the ulcer patients and 52% of the patients without ulcer (p less than 0.05). CP was demonstrated in 83% of the histologically diagnosed cases of gastritis (all grades) but no CP was detectable in patients with normal gastric mucosa. Among the ulcer patients, CP was more frequent in those with no history of medication with nonsteroidal antiinflammatory drugs (p less than 0.01). Sera from CP-positive ulcer and gastritis patients have significantly higher levels of IgG antibodies to CP than sera from those found to be free of ulcer or gastritis. In 200 blood donors an increasing percentage of elevated CP-antibody titres were found with advancing age (50% over 60 years of age). Quantification of circulating CP antibodies, would thus seem a valuable adjunct in the diagnosis of gastritis and probably also of peptic ulcer. The data presented furnish further evidence of the high rate of association of CP and the gastritis-peptic ulcer complex.