We herein report a case of early primary duodenal carcinoma arising from Brunner's glands synchronously occurring with sigmoid colon carcinoma. A 65-year-old man with a 5-year history of diabetes mellitus and benign prostatic hypertrophy was admitted to our hospital to undergo a resection of sigmoid colon carcinoma in December 2000. Upper gastrointestinal endoscopy, which was performed as routine preoperative screening, revealed an elevated submucosal-tumor-like lesion with a shallow central depression in the anterior wall of the duodenal bulb. A partial duodenectomy with a partial gastrectomy including No. 5 and No. 6 lymph node dissection and a sigmoidectomy were thus performed. The patient's postoperative course was uneventful. The histopathology of the resected duodenal specimen revealed the tumor to be an adenocarcinoma arising from Brunner's glands. The patient has remained disease-free and has shown no relapse for 6 years postoperatively. Because duodenal carcinoma arising from Brunner's glands is very rare, we report our case with a review of 25 similar documented cases.