Familial adenomatous polyposis coli (FAP) is an autosomal dominant genetic disorder caused by mutations of the APC gene on the long arm of chromosome 5. While multiple colorectal adenomas usually developing in early adolescence represent the most conspicuous phenotypic feature, the disease represents a generalized hyperproliferative disorder with various extracolonic manifestations. Duodenal cancer and desmoids are the leading causes of death in FAP patients after prophylactic colectomy. The prevalence of duodenal adenomatosis among FAP patients varies from 50% to greater than 90%, while only few patients (3-5%) develop duodenal cancer. Periampullary adenomas seem to carry a high risk of malignant transformation. The sensitivity of endoscopic procedures for early detection of severely dysplastic or malignant duodenal lesions is low, and the prognosis of duodenal cancer is poor. Thus the question arises whether it is possible to define a subgroup of high-risk patients for duodenal malignancy, and whether severe duodenal adenomatosis should lead to an aggressive prophylactic surgical approach. This contribution discusses the current literature and presents the experience of the Heidelberg Polyposis Register based on gastroduodenoscopy findings in 231 FAP patients. In 135 cases (58.4%) macroscopic duodenal adenomas were observed. The majority of patients displayed numerous lesions throughout the duodenum, while adenomas were restricted to the periampullary region in ten. Four patients suffered from duodenal cancer. Twenty-two required an endoscopic or operative intervention. Five were treated by laparotomy and duodenotomy, while ampullary excision was indicated in six cases. Eight patients underwent partial pancreaticoduodenectomy for severe duodenal adenomatosis.