Germ cell tumors make up about 20% of mediastinal tumors and cysts. They are thought to arise from extragonadal germ cells. They are always associated with the thymus gland and may even be fully contained within it when small. Mediastinal germ cell tumors are not related to thymomas. The possibility of metastases from testicular or ovarian primary tumor must be excluded before a diagnosis of primary mediastinal germ cell tumor is made. There is an association between mediastinal germ cell tumors and Klinefelter syndrome. The histologic subtype of mediastinal germ cell tumor present is related to the patientís sex. Seminomas are seen only in males. Embryonal carcinoma, yolk sac tumor, teratocarcinoma, and choriocarcinoma show male predilection but may sometimes be seen in females. Mature cystic teratomas - the most common type of mediastinal germ cell neoplasm - occur with equal frequency in both sexes. Malignant non-seminomatous germ cell tumors of the mediastinum often show elevated serum beta-hCG and/or alpha-fetoprotein levels. Like their testicular counterparts, more than 80% of mediastinal germ cell tumors show amplification of 12p in the form of an isochromosome. Mediastinal non-seminomatous germ cell tumors have adverse prognosis (just like their testicular counterparts). In addition, they are more prone to complications such as development of somatic-type malignancies, including adenocarcinoma, neuroendocrine tumors, malignant peripheral nerve sheath tumors, rhabdomyosarcomas, angiosarcomas, and hematologic neoplasms (acute leukemias, systemic mast cell disease).