We report a case of coexisting poorly differentiated endocrine carcinoma and conventional adenocarcinoma in the ampulla of Vater. A 70-year-old female had a recent history of symptoms and signs related to obstructive jaundice. An initial endoscopic biopsy of the ampulla of Vater showed a poorly differentiated endocrine carcinoma in the lamina propria of duodenal mucosa. The tumor could also be categorized into large cell neuroendocrine carcinoma under the WHO classification of pulmonary neuroendocrine tumors. The patient underwent Whipple's operation. After thorough microscopic examination of the ampulla of Vater, we incidentally found another conventional adenocarcinoma on the inner side of the duodenal papilla, and the tumor collided with the aforementioned carcinoma. The association of neuroendocrine tumor and adenocarcinoma has been reported in a few case reports and a small series. We also review the literature concerning large cell neuroendocrine carcinoma in this area.