As with most endocrine tumors, the malignant potential depends on evidence of local or distant invasion (metastasis), so it is important to differentiate synchronous/metachronous endocrine tumors from their metastases. A 90-year-old man was operated due to tumor of the ampulla of Vater. As the surgical specimen was examined macroscopically, a second tumor focus, measuring 1 cm in diameter, was detected at the duodenum. There were no clinical syndromes due to hormone hypersecretion. Microscopically, the ampullary tumor had trabecular and rosette-like patterns, with many necrotic areas. It had invaded the muscularis mucosa at the duodenal wall. The latter duodenal tumor was located in the submucosa and had distinct borders. This tumor consisted of trabecular structures with stroma rich in lymphoid aggregates. Immunohistochemistry revealed positivity for synaptophysin and gastrin and negativity for somatostatin. In addition, the whole antral portion of the Whipple resection material showed diffuse parietal cell hyperplasia. The tumors were diagnosed as well-differentiated endocrine carcinoma in the ampulla of Vater according to the WHO classification 2000, a gastrin-producing well-differentiated endocrine tumor in the first portion of the duodenum without regional lymph node metastases, and a diffuse parietal cell hyperplasia at the antral portion of the stomach. In conclusion, clinical findings and the postoperative diagnosis suggest that this patient had primary synchronous neuroendocrine tumors of the small intestine.