Upper gastrointestinal endoscopic examination of 40 patients on immunosuppressive therapy after successful kidney transplantation revealed a duodenal ulcer in four cases and duodenal erosions in six. Between four and six biopsies were collected from the antral and the fundal mucosa in each case and from the duodenal mucosa in 29. Antral superficial gastritis was found in 13 cases and fundal superficial gastritis in three. In the fundus, heterotopic calcifications were seen in eight cases and cytomegalovirus inclusion bodies in two. Parietal cells were increased in number and reached the pyloric zone in 34 cases, whilst they spread within the duodenal mucosa in 12. Multinucleate parietal cells were of common occurrence and in some cases showed mitotic figures. Duodenitis was found in 16 cases and in duodenal mucosal cells cytomegalovirus inclusion bodies were seen in nine cases. Biopsy evidence of hyperplasia of Brunner's glands was encountered in 26 cases. Of 20 patients tested for gastric acid secretion, hypersecretion occurred in 14 and normal secretion in six. Endoscopic and histological changes relate to the secretory pattern. Cytomegalovirus inclusion bodies relate to immunosuppressive therapy. An inverse temporal relationship exists between heterotopic calcification and the duration of renal transplant. Steroids seem to be responsible for gastroduodenal changes in transplanted patients.