Since Helicobacter pylori infects the gastric mucosa in most patients with chronic duodenal ulcer, infection with this organism has been implicated in the pathogenesis of this common disease. We postulated that if H. pylori is pathogenic in the usual type of duodenal ulcer, it should be less common when duodenal ulcer has another, specific etiology, such as Zollinger-Ellison syndrome. Gastric mucosa was compared from 18 patients with proven Zollinger-Ellison syndrome (17 of whom had had duodenal ulcer disease) and 18 controls with chronic duodenal ulcer without such a diagnosis. All subjects, who were matched for age and sex, had undergone elective gastric resections. Gastric tissues were stained by hematoxylin-eosin and Giemsa and were reviewed by an experienced pathologist who was unaware of the diagnosis. The frequency of H. pylori in patients with Zollinger-Ellison syndrome (8/18) was lower than in controls with duodenal ulcer (16/18; P less than 0.02). Moreover, chronic antral gastritis scores were higher in patients with duodenal ulcer (P less than 0.01). In Zollinger-Ellison syndrome, peak acid output was lower in patients positive (median 22 meq/30 min) compared to those negative for H. pylori (median 32 meq/30 min; P less than 0.02) but serum gastrin was correspondingly lower in patients positive for H. pylori (P less than 0.05). H. pylori infection appears to be more frequent when duodenal ulceration is not associated with another etiology, such as acid hypersecretion in Zollinger-Ellison syndrome. H. pylori infection in Zollinger-Ellison syndrome may also be associated with decreased gastric acid secretion.