An article describes diagnostic difficulties in patient with giant renal cyst, erroneously diagnosed as ascites on ultrasonographic examination. Patient was initially suspected to have disseminated intraabdominal malignancy. Abdominal paracentesis of supposed ascites was performed. The diagnosis of giant renal cyst was finally made by CT and patient was treated surgically. The limitations of ultrasonographic examination are pointed out are and a brief review of similar cases is given.