Germ cell tumors make up about 20% of the mediastinal tumors and cysts. They are thought to arise from extragonadal germ cells. Their location is related to the thymus and may be completely encased within it when small. The possibility of metastases from gonadal primary must be excluded. Some cases of mediastinal germ cell tumors are associated with Klinefelterís syndrome. The most common type seen is mature cystic teratoma (shown here). The specimen has several large cysts surrounded by adipose tissue. The cysts were filled with mucoid fluid and lined by intestinal type epithelium.