A 26-year-old man with left inguinal pain and frequent urination was examined. An abdominal ultrasound revealed a cystic lesion. In further examinations, CT and MRI showed a large cystic lesion of about 20 cm in size, connected to mesenteric- derived blood vessels. We suspected a huge mesenteric lymphangioma and decided to perform a laparotomy. A tumor was seen in the mesentery of the jejunum and adhered to the duodenum widely. The tumor could be removed safely without resection of the duodenum by first sucking the contents and shrinking the tumor. The final pathological diagnosis was mesenteric lymphangioma. Adult mesenteric lymphangiomas measuring larger than 20 cm are relatively rare. We review the case in the context of the relevant literature.