A case of removal of a fetus in fetu in a 47-year-old man is reported. The patient had an upper abdominal mass since birth that had never caused any subjective symptoms. A preoperative computed tomographic scan was useful to confirm the diagnosis. The operative specimen consisted of a cystic mass about 20 cm in diameter, situated in the upper retroperitoneal space. The cyst was full of a yellowish fluid and hairs. A bony structure, about 10 cm in diameter, contained a vertebral axis connected to the ribs and was adherent to the cystic wall. To our knowledge this is the first reported case of fetus in fetu described in an adult man. The tumor, present for 47 years, did not grow or cause any complications and did not show any sign of malignancy.