Double fetus in fetu: diagnostic imaging.

Fetus in fetu is a rare pathological condition, presenting as a congenital tumor. It consists of a malformed parasitic twin that is found within the body of its sibling. Less than 70 cases have been reported and in most of them a definite diagnosis was only made during surgery. We present a case with two fetuses in the retroperitoneum and describe the criteria to be kept in mind for a correct preoperative diagnosis using the current imaging modalities.