Immature teratoma can be viewed as intermediate between mature teratoma and embryonal carcinoma. Unlike the mature teratoma, elements of the three perm cell layers are incompletely differentiated and not arranged in organoid fashion In some area, more mature forms of these tissues may also be seen, Although this tumor is clearly malignant, they may not display clear-cut cytologic features of malignancy. The clinical prognosis is poor. We have experienced one case of recurrent immature teratoma. In first operation[1987, April], tumor of anterior mediastinum was removed with wedge resection of ant. segment of RUL % med. segment of RML. In 2nd operation[1990, June], recurrent teratoma of right inner thoracic wall was resected with partial 5th rib resection and wedge resection of lat. segment of middle lobe. Two months later, the 3rd operation[1990, September] was done, which was a removal of mass on thoracic wall near sup, segment of RLL and partial rib resection of 3rd, 4th & 5th. In November 1990, last operation[4th operation] was made. It was enucleation of walnut sized tumor located between medial segment of RML, and 4th. intercostal space, well encapsulated with endothoracic fascia and invaded into lung parenchyme, Adjuvant chemotherapy was done after each operation, but radiotherapy[5000 Rad] was done only after 1st operation. There was no evidence of recurrence after last operation. The patient is well-being still now,