A five-year-old girl had been noted, since birth to have abdominal distension. There was no history of twinning or teratoma in the family. Physical examination discovered a 15 x 10 cm mass in the abdomen. Roentgenologic examination demonstrated the presence of an axial skeleton and long bones within the mass. MRI showed a retroperitoneal mass with components of cystic, bony and fatty tissues. When the mass was surgically removed from the retroperitoneal cavity, pathologic examination disclosed a parasitic fetus measuring 15.3 x 6.3 x 6.0 cm in size and suspended by a peduncle within a capsule. The fetus was covered with skin, sebaceous materials and black hair. There were bilateral and symmetric upper extremities with a flapper-like, deformed hand. No obvious genital organ was noted. Dissection of the midline, revealed a vertebral column with spinal cord. An atrophic skull bone containing cephalized glial tissue was noted. A thoracic cavity and scapular bone were found in the thoracic cage; however, no heart was noted. Gastrointestinal tract and kidney-like organs were seen within the abdominal cavity. Microscopically, there were glial tissues within the skull bone, spinal cord and vertebra. The upper extremities revealed two long bones surrounded by myxomatous and fibrous tissue. A well developed intestine and an ovary containing follicles with nearby lympy nodes were seen. The capsule showed fibrous tissue with calcification and hairs. It was distinguished from the teratoma by the presence of a vertebral axis and by an appropriate arrangement of other organs or limbs with respect to the axis, and corresponding to the generally recognized diagnostic criteria of fetus in fetu.