Helicobacter pylori is associated with peptic ulcer, and a causal relationship has been postulated. We investigated the association between Helicobacter pylori and gastric acid output. Two hundred forty-one patients were studied: 173 with duodenal ulcer, 51 with gastric ulcer (41 corpus, 10 prepyloric), and 17 with combined gastric and duodenal ulcer. In 194 patients (80%), Helicobacter pylori could be demonstrated histologically from gastric antral biopsies. The presence or absence of Helicobacter pylori was not influenced by age, sex, or use of tobacco or analgesics. Patients with duodenal ulcer or combined gastric and duodenal ulcer had similar gastric acid outputs irrespective of the presence or absence of Helicobacter pylori. However, gastric ulcer patients with Helicobacter had higher basal and maximal acid outputs when compared to patients without Helicobacter (mean basal output: 4.1 mmol/hr vs 2.4, P less than 0.05; mean maximal output 19.5 mmol/hr vs. 14.4, P less than 0.05). Although Helicobacter pylori is associated with both gastric ulcer and duodenal ulcer, its significance may be different in the two diseases.