Among anterior mediastinal tumors affecting male patients of around 20 years old, mediastinal malignant teratoid tumor must be considered as one of the possibilities. Malignant teratoid tumor can be classified as seminoma, non-seminoma or mixed, according to clinical behavior. In the non-seminoma group, AFP and/or HCG can be the specific markers in the diagnosis or assessment of the effect of treatment. Chemotherapy with CDDP must be the first choice of treatment in these types of tumor, just as chemotherapy is the first choice of therapy in the treatment of small cell lung cancer, and surgery must be the adjuvant treatment to chemotherapy. On the other hand, seminoma can be treated by surgery and radiation, which has been proven to yield a good prognosis. In cases of seminoma which produces HCG and/or AFP, chemotherapy with CDDP must be added to surgery and radiation as in non-seminomatous mediastinal teratoid tumors, because the production of such markers in seminoma is considered to be one of the poor prognostic factors in the treatment of seminomatous mediastinal teratoid tumors.