Germ cell tumors of the mediastinum can be divided into three categories: 1) mature teratomas which have all elements at a mature level; 2) immature teratomas which are similar to the mature teratomas, but also contain immature epithelial or mesenchymal elements or blastema; and 3) embryonal tumors which contains elements which are recognized as dysgerminoma, embryonal carcinoma with or without yolk sac elements, and choriocarcinoma. Immature teratomas are the rarest type, accounting for only about 1% of mediastinal teratomas. Two cases of immature teratoma of the mediastinum occurring in infants are reported. One tumor was completely excised. The child is well without evidence of disease two years later. The other immature teratoma was unresectable. Biopsy showed it to be similar to the teratoma that was excised. No postoperative treatment was given. The tumor has not changed appreciably in size, but the child has grown normally for six years so that the tumor mass, which initially filled his chest, is now evidenced as mediastinal widening. A review of the reported cases of immature teratomas in the mediastinum shows that the prognostic value of the histologic appearance of these tumors has not been developed to the same degree as it has for teratomas in the ovary or the sacrococcygeal region. The immature teratomas that occur in infants behave as mass lesions as do the mature teratomas. Immature teratomas in the mediastinum of children in their late teens and in young adults behave as highly malignant tumors similar to the embryonal carcinomas.