A 3.5-month-old boy was hospitalized because of an abdominal mass found accidentally. On physical examination, a smooth, firm, nontender mass was present in the right upper quadrant. Abdominal ultrasonography revealed a large, hyperechoic, heterogenous mass with clear boundaries, and scarce blood flow. Abdominal CT scan showed a bulky right retroperitoneal mass. Three-dimensional CT imaging demonstrated spine, iliac bone, and long bones of limbs. The mass was excised successfully. After opening the sac it was noted to contain an incompletely developed fetus with grossly visible limbs, clearly discernible male genitalia, hairs, and a poorly formed head. The fetus was connected to the sac via an 8 cm cord-like structure. Microscopic examination of the mass revealed the presence of skin, cartilage, bone, intestine, and cysts with simple cuboidal epithelium. The use of CT scans enhanced the accuracy of pre-operative diagnosis. Identification of the vertebral column and the long bones of limbs are important indications for the diagnosis. Pathologically, fetus in fetu has many characteristics different from teratoma.