Helicobacter pylori affects gastric secretion. This functional effect might have a morphometric counterpart. Therefore, the gastric cell secretory compartment was morphometrically assessed in different pathophysiologic conditions related to Helicobacter pylori infection. Nineteen Helicobacter pylori-positive nonduodenal ulcer subjects, 15 omeprazole chronically treated subjects, and 19 duodenal ulcer patients were studied against 19 controls. Somatostatin, gastrin, enterochromaffin-like, and parietal cell density was assessed in gastric biopsies. No differences in any cell type density were found between Helicobacter pylori-positive nonduodenal ulcer subjects and controls. On the contrary, differences were significant when comparing omeprazole and duodenal ulcer patients to controls (higher density of gastrin, enterochromaffin-like, and parietal cells, lower density of somatostatin cells). In duodenal ulcer a reversion to control values followed Helicobacter pylori eradication and ulcer healing. A direct linear correlation between enterochromaffin-like, gastrin, and parietal cell density was demonstrated. An almost complete map of mucosal cells involved in gastric secretion is provided by this study. The cell density pattern, identical to the omeprazole group, points to an impaired feedback control of secretion in duodenal ulcer. The reversion to control values after Helicobacter pylori eradication and ulcer healing demonstrates the pathogenetic role of Helicobacter pylori-host interaction in these changes.