In a prospective study, 20 patients with endoscopically proven duodenal ulcers were randomised to be treated with cimetidine 1 g daily or with antacids 350 mmol daily. The duration of treatment was 30 days, but this was extended to three months in 3 patients in the cimetidine group and 4 patients in the antacid group. A morphological study of biopsies taken via a gastroscope from the corpus mucosa showed no change in mucosal height, parietal cell density per unit volume or changes in cellular infiltration after 30 and 90 days treatment in the two groups. Neither H-2 receptor blockers nor antacids alter mucosal height or parietal cell density. Treatment was not found to induce gastritic changes in the mucosa.