Cancer of the upper gastrointestinal tract is a leading cause of death in patients with familial adenomatous polyposis who have previously undergone total colectomy. The prevalence and the degenerative risk of gastric and duodenal adenomas has been well documented, but little is known about the occurrence of jejunal polyps in these patients. We evaluated 21 (16 affected and five high-risk) subjects in nine families prospectively in order to assess the frequency of polyps in the upper jejunum. With the push-type technique, it has been possible to insert a standard gastroduodenoscope or a longer fiberscope 15–80 cm beyond the ligament of Treitz. Eight out of 16 (50%) affected patients and none of the non-affected subjects had polyps. All lesions were adenomas, including a 4-cm tubulovillous adenoma with severe dysplasia, and were almost exclusively located in the first 20 cm of jejunum. The depth of insertion obtained with gastroduodenoscopes was significantly smaller than that obtained with longer endoscopes (pediatric colonoscope or experimental enteroscope). However, the jejunal area thought to be at the highest risk of adenoma (the proximal 20 cm) was usually investigated even with standard 105-cm long gastroduodenoscopes. This study confirms that the upper jejunum has a high prevalence of adenomas in patients affected by familial adenomatous polyposis and that proximal jejunoscopy is a worthwhile, not excessively uncomfortable procedure that should be added to the usual follow-up protocols.