We herein describe the case of a 48-year-old man who presented to our hospital with abdominal distension and pain. Preoperative studies including abdominal ultrasonography and computed tomography failed to determine the cause of the pain. At laparotomy, a giant cystic tumor of the small bowel mesentery was found. Histologically, the tumor was diagnosed as a cystic lymphangioma. Although mesenteric lymphangiomas are rare, especially in adults, they should be considered as a possible cause of acute abdomen.