Variable endoscopic surveillance protocols and treatment strategies have been proposed for periampullary neoplasia in familial adenomatous polyposis (FAP), primarily because of the lack of long-term, prospective natural history data. A total of 115 patients with FAP were followed prospectively for 10 years with periodic side-viewing upper gastrointestinal endoscopy by a single surgeon. The appearance of the duodenum was classified as stages 1 to 5. Statistical analysis included one-way analysis of variance for age comparisons between stage groupings and Kaplan-Meier analysis for the lifetime risks of having a particular stage of duodenal polyposis. Eighty-seven patients had multiple endoscopies over an average of 6.6 years. Thirty-three subjects had a change in stage, within an average time of 3.9 years at an average age of 41 years. The risk of having stage 3 or 4 duodenal neoplasia increased exponentially after the age of 40. The degree of dysplasia did not correlate with stage at initial classification. Progression of neoplasia in the duodenum of patients with FAP is slow. The severity of duodenal polyposis increases with age and is not influenced by the initial stage. The average time for progression of adenoma to carcinoma is likely long.