Abstract: A Japanese woman with familial adenomatous polyposis in whom a duodenal ampullary adenoma underwent malignant change during a 10-year follow-up period is reported. After restorative proctocolectomy in 1989, and extensive small bowel resection for desmoid disease in 1991, regular surveillance duodenoscopies, including three to nine biopsies (mean, 4.8) were performed annually or biannually. Until 1995, the endoscopic findings of duodenal polyposis (including an ampullary polyp) did not progress and the histopathology did not worsen. In 1996, there was an increase in the number and size of the duodenal polyps, and the ampulla of Vater looked enlarged. Open surgery was discussed but not proceeded with because of the risk for short bowel syndrome. In January 1998, she was admitted with a diagnosis of acute pancreatitis. Duodenoscopy and radiological examination revealed that an advanced ampullary cancer had developed, and histopathology revealed a well-differentiated adenocarcinoma. Multiple hepatic metastases and ascites led to her death, in June, 1998. This in-vivo demonstration of the adenoma-carcinoma sequence highlights current limitations in the surveillance and treatment of duodenal lesions.