102 patients with familial adenomatous polyposis underwent upper gastrointestinal endoscopy as a screening test for gastroduodenal adenomas. 100 had duodenal abnormalities (dysplasia in 94, and hyperplasia in 6), usually in the second and third parts of the duodenum (91%). The periampullary area was abnormal in 87 of 97 patients who had a biopsy specimen taken from this site (dysplasia 72, hyperplasia 13, and inflammation 2). By contrast, gastric dysplasia was found in only 6 patients. Classification of duodenal polyposis on a 5-grade scale (stages 0-IV), based on polyp number, size, histology, and severity of dysplasia, showed that 11 had stage IV disease: these patients are at greatest risk of malignant change and require close surveillance. The pattern of dysplasia observed in the upper gastrointestinal tract resembled the pattern of mucosal exposure to bile.