We report a case of retroperitoneal cystic lymphangioma in an 65-year-old male. Two years prior to this admission, a mass was incidentally detected by ultrasound during hospitalization for the treatment of lung cancer. Ultrasound study showed a multiloculated cystic mass with debris and CT revealed a fat-fluid level within the mass. On two years follow-up, the mass has changed into a unicameral mass without internal echogenicity on US and homogeneous water-density mass on CT. The mass was removed by surgery and confirmed as retroperitoneal cystic lym-phangioma.