The purpose of this paper is to define a prenatal diagnostic clue for fetus in fetu (FIF) based on the characteristic findings of four FIF cases studied by the authors. A retrospective analyses of prenatal ultrasonography (US), postnatal US, computed tomography (CT) scans and operative findings were carried out on each of the four FIF cases collected from a multi-center. Prenatal US findings for each of the four cases showed a fluid-filled sac with a solid portion "floating" within it. In addition, bony structures were found in the solid portion. In each of the four cases, the postpartum imaging studies were consistent with the prenatal US findings. Operative findings revealed a solid mass within a fluid-filled sac surrounded by a transparent membrane. The solid mass was connected to the membrane by a stalk. Histopathologically, the cells making up the sac were of the same type as those of the amnion, while the solid mass had the general characteristics of FIF. In conclusion, FIF can be suspected when prenatal US shows a solid mass with bony structures within a fluid-filled sac in a newborn.