Germ cell tumor of the testis is the most common solid tumor malignancy in young adult men.1 Studies show a six-to tenfold increase in germ cell tumors in patients with a first degree relative with germ cell tumor, 2 demonstrating a strong familial and genetic component. Mature teratoma is present in approximately 50% of mixed germ cell tumors (MGCT) but is uncommon as a "pure" germ cell tumor in post-pubertal patients, supporting the belief that teratoma is mostly derived from other forms of invasive germ cell tumor of the testis. Other components frequently seen in MGCT include immature teratoma, seminoma, embryonal carcinoma, yolk sac tumor, and choriocarcinoma.Teratomas are histologically diverse tumors that contain a variety of tissues, including elements derived from ectoderm (squamous epithelium, neural tissue), mesoderm (muscle, cartilage, bone), and endoderm (intestinal, pancreatic, respiratory). Mature teratomas are usually composed of differentiated tissues from two or three of these germ cell layers. They occur in both children and adults, but the behavior in these groups is different, with the post-pubertal cases having associated metastases but not the pre-pubertal. Studies have shown that 22 to 43% of adult patients with pure teratoma may have metastases during their clinical course 3-5 and there is commonly coexisting intra-tubular germ cell neoplasia of the unclassified type in the testis. 3,6 The metastatic lesions in such cases may be comprised of either pure teratoma or other components of MGCT.3,5 Regardless of whether these metastatic lesions are the direct metastasis of a primary teratoma or the teratomatous differentiation of a more primitive component after metastasis, it is evident that primary pure teratomas of the testis have metastatic potential, and should be considered malignant. These patients typically do well with complete surgical excision of the metastatic deposits. [7][8][9]