Fetus in fetu associated with an undescended testis.

Fetus in fetu is an extremely uncommon cause of an abdominal mass in the neonate; fewer than 30 generally accepted cases are recorded in the literature. We report a case of intraabdominal fetus in fetu, with a unique location within an undescended left testicle. Chromosomal studies of cells from the fetus in fetu and the surviving infant revealed identical 46,XY karyotypes.