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Primary Mesenteric Carcinoid Tumor Presenting with Carcinoid Syndrome. Neuroendocrine neoplasms (NENs) are a diverse group of tumors arising throughout the body with a common origin from neuroendocrine cells. Well-differentiated NENs, also known as neuroendocrine tumors (NETs), are generally indolent and are often found incidentally, while poorly differentiated tumors are more aggressive. Carcinoid tumors are NETs arising from the gastrointestinal tract and less commonly from the lungs, thymus, and kidneys. NETs in the mesentery arise from metastasis from primary tumor, and carcinoid syndrome in this setting results from concomitant metastasis to the liver. Primary mesenteric carcinoid tumors are very rare. We present a 64-year-old man with carcinoid syndrome from a mesenteric carcinoid tumor without evidence of liver metastasis or other primary tumor sites.