Cystic lymphangiomas of the small bowel mesentery are rare manifestations of intra-abdominal tumors. Usually, they are discovered incidentally during examination for an unrelated abdominal illness. We present a case of a 4-year-old boy who was admitted to our hospital because of the right lower quadrant acute abdominal pain suspect of acute appendicitis. At laparotomy, a giant, cystic, encapsulated and lipomatous mesenterial mass was found, 15 x 15 x 10 cm in size, infiltrating the jejunum. The tumor was located 70 cm from Treitz's ligament. Extirpation of tumor mass with intestinal resection of the involved loops was necessary. Pathologic examination confirmed the diagnosis of mesenteric cystic lymphangioma. Although they are rare, cystic mesenteric lymphangiomas should be considered as a possible cause of acute abdomen and treated with surgical resection. Prognosis after surgical removal is excellent.