We report a case of omental cyst in an 11-month-old infant. The cyst occupied nearly the whole abdomen and mimicked massive ascites on sonography. The correct preoperative diagnosis was achieved through the use of CT, and partial torsion of a huge omental cyst was found at surgery. The diagnostic role of sonography and CT in ascites of unknown etiology is emphasized. VVC 2006 Wiley Periodicals, Inc. J Clin Ultrasound 35:85–87, 2007; Published online in Wiley InterScience (www.interscience.wiley.com).