Mesenteric neuroendocrine tumors are usually metastases originating from the small intestine; however, primary mesenteric cases are rare. We present an interesting case of a mesenteric neuroendocrine tumor that changed its internal composition from cystic to solid. A 72-year-old male visited our hospital because of epigastralgia 4 years earlier. A 25-mm tumor was recognized around the terminal duodenum on computed tomography and magnetic resonance imaging, and was diagnosed as a cystic lesion. Over the following 2 years, the tumor grew to 40 mm and its internal composition changed from cystic to solid. The lesion showed positive findings on fluorodeoxyglucose positron emission tomography. Upon laparotomy, a solid tumor was detected in the mesentery of the jejunum near the ligament of Treitz. The tumor was extracted without intestinal resection and was diagnosed as a low-grade neuroendocrine tumor after histopathological and immunohistochemical examination. One year has passed since the operation, and there has been no recurrence.