An 11-year-old boy entered our hospital with severe lower left abdominal pain. Ultrasonographic examination with a 7.5 MHz probe revealed a multilocular cystic mass (8×6×4 cm) in the lower left lateral abdomen. The lower descending colon was compressed, but had not been infiltrated by the cystic mass. A cystectomy was performed. The resected mass was a multilocular mesenteric cyst. Histopathologically, this tumor was diagnosed as lymphangioma. Ultrasonography was very useful in detecting this mesenteric cyst.