We report imaging findings in a case of idiopathic localized dilatation of the ileum (ILDI). CT revealed a large cystic mass containing air in the pelvic cavity. Sonography showed a cystic lesion with internal echo in the pelvic cavity. In addition, slow peristaltic movement was seen in the lesion. Small bowel series showed a bilobular dilatation of the ileum with two narrow transitions connecting the proximal and distal bowel loops.