Although the double wall or muscular rim sign is considered to be a characteristic sonographic imaging finding of an enteric duplication cyst, this sign can also be produced by Meckel's diverticulum and sonographic artifacts. The Y-configuration on in vitro sonography has been reported to be useful for the definitive diagnosis of an enteric duplication cyst. Here, we report a case of an intraoperatively confirmed ileal duplication cyst that was correctly diagnosed before by identification of the Y-configuration on in vivo sonography.