Endoscopic biopsies were taken from the oxyntic mucosa in patients with duodenal ulcer (DU) before and after 4 weeks of treatment with omeprazole. Biopsies were also obtained from Zollinger-Ellison (ZE) patients before and during treatment with omeprazole; these patients had previously failed to respond to other therapies aiming at reducing gastric acid secretion. Healthy volunteers served as controls. Light-microscopic studies of biopsy sections revealed superficial gastritis in most of the patients. Endocrine cell volume density (that is, the percentage of mucosal volume occupied by endocrine cells) was 0.3% in the DU patients and 0.7% in the ZE patients before omeprazole treatment. No significant change occurred during omeprazole treatment, which in the ZE patients continued for up to 21 months. In the controls, 0.3% of the mucosal volume was occupied by endocrine cells. Parietal cell volume density was about 15% in both the DU patients and in the controls; in the ZE patients, slightly but not significantly higher values were recorded. No significant change was observed during omeprazole treatment.