A 81-year-old man with 2 days of nausea, vomiting, and mild abdominal pain and a medical history of an abdominal aortic aneurysm (AAA) diagnosed 5 years earlier was admitted as an emergency patient because of these symptoms. Computed tomography showed duodenal compression and a markedly dilated stomach and oral duodenum with an intact 7.5cm infrarenal AAA. The patient was diagnosed with a duodenal obstruction caused by the aneurysm. Elective graft replacement of the aneurysm was conducted successfully after gastric decompression. He made an uneventful recovery and remains well in follow-up. AAA is a rare cause of duodenal obstruction. We report the unique cause of duodenal obstruction secondary to expansion of an infrarenal AAA. ©2007 The Japanese Society of Gastroenterological Surgery.