Fetus in fetu or differentiated teratomas?

Two cases of congenital teratoma were operated upon. In view of the high degree of organoid differentiation of the teratomas with rudimentary limbs, intestine, brain-like and pulmonary tissues, it was difficult to distinguish it from fetus-in-fetu. In the light of the data obtained and extensive review of related literature, we consider that fetus-in-fetu and teratoma may not be unrelated entities. Our cases support the view held that fetuses in fetu are highly differentiated teratomas.