Upper gastrointestinal (UGI) endoscopy was performed in 35 asymptomatic patients with familial adenomatous polyposis (FAP) to assess the prevalence of gastric and duodenal polyps and to demonstrate efficacy of endoscopic surveillance in patients with FAP. UGI polyps were found in 25 (71%) of 35 polyposis patients. Among these, gastric fundic gland polyps were involved in seven patients, and UGI adenomas in 18 patients; there were nine gastric adenomas and 14 duodenal adenomas, including six cases of duodenal cancer. UGI polyps were found in 13 of 15 patients with extracolonic manifestations (87%), but in only 12 of 20 patients (60%) without. The median follow-up interval was 8.7 years in growing duodenal adenomas, but 3.5 years in adenomas without changes in size and number. It was concluded that surveillance gastroduodenoscopy every 3-5 years might be enough to treat duodenal adenomas by endoscopic mucosal resection.