Foetus-in-fetu (FIF) is a rare congenital condition in which a vertebrate foetus is incorporated within its twin. The authors report the case of a newborn girl with prenatal ultrasonographic diagnosis of an intra-abdominal mass. Abdominal ultrasonography and computed tomography of the abdomen showed a heterogeneous cystic mass containing multiple calcifications. The patient had a laparotomy at 18 days of age with excision of a well-encapsulated 6×5 cm retroperitoneal mass, containing many organs. Anatomicopathologic examination showed a relatively well-differentiated FIF attached to an amniotic sac by a rudimentary umbilical cord. Encephalon, coroidal plexus, vertebral bodies, rudimentary limbs, thyroid gland and teeth were identified. The postoperative period was uneventful.