Fetus-in-fetu is an extremely rare enigmatic condition, every case of
which should be carefully examined, particularly for the distinction between it and a
mature teratoma and the determination of its embryopathogenesis. This report deals
with a case of fetus-in-fetu that was found in two parts. The host(autosite) was a 14month-
old boy who first presented with a back mass. A large round mass, 7 X 6 X 6cm,
was removed from the left upper retroperitoneum. The mass was cystic containing 200ml
of yellow serous fluid and consisted of two separate masses. One part resembled a
stunted dysmorphic fetus, while the other was an amorphous lump, which disclosed
a sacrococcygeal bone with five vertebrae. Histologically, both masses showed largely
regressed digestive, respiratory, and musculoskeletal tissue. However, the parenchymal
organs were missing. Definite vertebral bones, intervertebral discs, and spinal cord
remnant were found
