BACKGROUND: Patients with genetic adenomatous polyposis syndromes have an increased risk for duodenal cancer, and clear surveillance recommendations exist for this group. However, limited data are available on the duodenal phenotype of patients with multiple colorectal adenomas (10-99) without a germline APC or MUTYH mutation. OBJECTIVE: We aimed to assess the frequency, extent, and progression of duodenal adenomas in patients with multiple colorectal adenomas without a germline APC or MUTYH mutation. DESIGN: This was an historical cohort study. SETTINGS: This study was undertaken at 2 polyposis registries: the Academic Medical Center in the Netherlands, and St. Mark's Hospital in the United Kingdom. PATIENTS: We collected data on all patients with 10 to 99 colorectal adenomas and absent APC and MUTYH mutations, who underwent ≥1 esophagogastroduodenoscopy. MAIN OUTCOME MEASURES: The frequency, extent, and progression of duodenal adenomas were measured. Demographic and endoscopic data were collected, described, and compared between patients with and without duodenal adenomas. RESULTS: Eighty-three patients were identified, of which 8 (9.6%) had duodenal adenomas, detected at a median of 58 years (range, 45-75 y). Duodenal adenomas were detected in 6 of 8 patients at first esophagogastroduodenoscopy. At diagnosis, all 8 patients had Spigelman stage I or II disease. Two of 5 patients with duodenal adenomas who underwent follow-up esophagogastroduodenoscopies increased to stage III disease. The other 3 remained stable. No one developed duodenal cancer. No differences in demographic and endoscopic data were found between patients with and without duodenal adenomas. LIMITATIONS: This study was limited by its retrospective design, selection bias, and small sample size. CONCLUSIONS: Duodenal adenomas are found in a minority of patients with multiple colorectal adenomas without a germline APC or MUTYH mutation, at an average age of 58 years, and, at diagnosis, disease severity is mild. These results are a first step in unraveling the duodenal phenotype of these patients, which is needed to provide appropriate upper GI screening and surveillance recommendations. See Video Abstract at http://links.lww.com/DCR/A357.