An endoscopic and biopsy study of the upper gastrointestinal tract in 102 patients receiving regular dialysis treatment (RDT) for chronic renal failure, showed prominent hypertrophic folds and a peculiar pale-grey or pale-pink colour of the gastric mucosa in most cases. Gastric erosions occurred in 11 cases and gastric ulcer in two. Duodenal erosions were found in 13 cases, eight of which were associated with hyperplasia of Brunner's glands. Duodenal ulcer occurred in one case. The histological findings were compared with those in 100 subjects with an endoscopically normal gastro-duodenal tract. In most cases of the RDT group the fundal area was enlarged at the expense of the antrum, fundal glands and/or isolated parietal cells often spreading within the duodenal mucosa. In most cases the duodenal bulb showed hyperplasia of Brunner's glands and heterotopic gastric surface epithelium. The frequency of the mucosal changes differs significantly in the two groups of subjects. Gastric hyperplastic changes in RDT are probably due to the trophic action of hypergastrinemia which is of common occurrence in such a condition, and can account for gastric acid hypersecretion. Acid hypersecretion, in turn, can account for the duodenal changes and gastroduodenal lesions.