Gastrointestinal neuroendocrine tumors arise from cells of the diffuse neuroendocrine system and can take place almost anywhere within the gastrointestinal tract. A 40-year-old man admitted to evaluate a duodenal subepithelial lesion which was incidentally found at health check-up. The polypoid duodenal subepithelial lesion, measuring about 7 mm, was removed by the endoscopic mucosal resection and the pathology confirmed a neuroendocrine tumor. Abdominopelvic computed tomography, done for staging work up, revealed a mass in the pancreatic head and the patient received pylorus preserving pancreaticoduodenectomy. Mass at the pancreas also found out to be neuroendocrine tumor but showed different histopathologic traits under immunohistochemical staining. The patient was also diagnosed as hyperparathyroidism and pituitary microadenoma. Finally, multiple endocrine neoplasia type 1 was confirmed, which was accompanied by duodenal neuroendocrine tumor.