Basal and pentagastrin-stimulated gastric acid secretion and basal serum gastrin level were investigated in 55 active duodenal ulcer patients with antral colonization with Helicobacter pylori (HP) and 17 patients without. Our study shows that basal (BAO) and pentagastrin-stimulated gastric acid secretion (MAO and PAO) were significantly higher in HP positive than in HP negative patients with duodenal ulcer disease. There were also a tendency to increase in basal serum gastrin concentration in HP positive patients. We suggest that antral HP increases antral gastrin release and gastric secretion. Increased acid secretion then causes duodenal ulcers by producing a low intraduodenal pH.