We describe a patient, 54-year-old woman, with lymphangiomas of the jejunum and mesentery presenting as acute abdomen. She had sudden onset of severe abdominal pain with guarding and fever. Physical examination revealed an elastic hard mass, with marked tenderness in the right abdominal region. Ultrasonography and computed tomography revealed a heterogenous mass, 20 x 15 x 8cm in size, in the mesentery. An emergency operation was performed with the diagnosis of diffuse peritonitis due to an inflammatory tumor in the mesentery. At operation, the tumor was found in the mesentery, adhering to a 60-cm segment of the jejunum. Multiple small tumors were seen in the submucosal layer of jejunum. The tumor was excised with a 60cm length of the jejunum. Histologically, the tumors were diagnosed as cavernous lymphangiomas. Although lymphangioma in the abdomen is rare in adults, we should consider it as one of the causes of acute abdomen.