We report a rare case of the growing teratoma syndrome (GTS). The patient was a 37-year-old man diagnosed as having a mixed germ cell tumor of the right testis. After resection of the right testis, the patient developed multiple lung metastases and chemotherapy was administered. However, in spite of the normalization of the serum AFP level, the lung metastases continued to increase in size. The lung metastases were successfully resected and pathological examination revealed the diagnosis of a mature teratoma. GTS defined as an enlarging tumor mass during or after chemotherapy in the case of germ cell tumor, with the normalization of the serum AFP. The total surgical resection of the mass yielded good result, and the recognition of this syndrome is important for successful treatment.