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.