As a pilot series on the frequency of gastroduodenal cytomegalovirus (CMV) involvement after liver transplantation, we examined forceps biopsies of 13 consecutive patients who underwent esophagogastroduodenoscopy during the first 3 months after transplantation. CMV was demonstrated in frozen sections by monoclonal antibody and immunoperoxidase staining. In parallel, peripheral blood was examined for CMV pp65 antigenemia. CMV antigens were detected in biopsies of ten patients, in ten cases in the duodenum and in four in the stomach. At the time of endoscopy, sic patients had CMV antigenemia, five of them had a simultaneous positive finding in the duodenum. Although all ten patients with the positive biopsy finding had some gastrointestinal symptoms, only one had severe enteritis. In liver transplant patients, CMV was commonly detected in leukocytes located in the mucosa of the upper gastrointestinal tract, especially in the duodenum. Further investigation is needed to determine the significance of positive CMV findings in the biopsies and their association with the development of severe gastrointestinal symptoms.