Gangopadhyay et al. Journal of Medical Case Reports 2010, 4:96 http://www.jmedicalcasereports.com/content/4/1/96 Page 5 of 7 Figure 4 Intra-operative picture of the fetus in fetu enveloped by a sac. Figure 5 Twin fetus in fetu connected by a cord-like structure. case, diagnosis was made pre-operatively through a CT scan; nevertheless abdominal ultrasound cannot be ignored in the initial evaluation of the anatomy of FIF. The twin fetus is typically suspended by a pedicle within a complete sac that contains fluid or sebaceous material. There is no placenta or chorionic villi at the point of attachment to the host [12]. In our case, our patient’s twin was present within the sac in his left retroperitoneum. The twin was also found suspended by a vascular pedicle to its host’s abdominal aorta, and multiple vascular attach- ments to the surrounding bowel were noted. It is impor- tant to note that the presence of a twin fetus in the host’s abdomen is extremely rare. Although the prognosis for FIF is more favorable than for cystic teratoma, the presence of immature elements nevertheless indicates the need for close clinical,