Fine structure of active and healed duodenal ulcer. In order to characterize the fine structure of active and healed duodenal ulcers, we examined tissue specimens of patients with active duodenal ulcer disease (n = 30) before and after treatment with either antacids (n = 16) or H2-receptor antagonists (n = 14), by light microscopy and various electron microscopic techniques, e.g., scanning and transmission electron microscopy. The characteristic histological feature of both the active and healed duodenal ulcer was the appearance of periodic acid-Schiff (PAS)-positive epithelial cells at the edge of the ulcers. Electron microscopy revealed that these cells were similar to a special type of mucus-secreting cell in the antrum (surface mucous cell). Their mucus granules contained mainly neutral glycoproteins. Helicobacter pylori were found attached to these cells in tissue specimens from 12 of 30 patients (40%). The mucous structure destroyed during the ulcerative phase regained its normal net-like structure after treatment. The ultrastructural healing process of duodenal ulcer was characterized by the presence of gastric metaplasia, by stunted microvilli of the duodenal epithelium (p less than 0.001 vs. control group), and an increased number of lysosome-like bodies (p less than 0.001 vs. control group) of the epithelial cells. These results were independent of the type of treatment, and showed that the repair mechanisms were incomplete after a 4-wk period of treatment. 