Omental cysts are the least common cystic lesions of the abdomen. Large intraabdominal cysts can mimic ascites. The incidence of 1 in 20000 pediatric hospital admissions makes them hard to diagnose. We report a three year-old boy with accidentally discovered intraperitoneal fluid collection on ultrasonography. The patient was scheduled for operative treatment with working diagnosis of ascites, but finding of no fluid in Morison’s pouch brought to massive cystic intraabdominal lesion as differential diagnosis. The cyst was emptied; altered parts of the greater omentum were completely excised. Final pathology confirmed lymphangioma. Explorative laparotomy led to correct diagnosis and proper surgical treatment.