proportions. Embryonal cell carcinoma Embryonal cell carcinomas, a more aggressive tumor than seminoma usually occurs in men in their 30s. Although it is the second most common testicular tumor after seminoma, pure embryonal cell carcinoma is rare and constitutes only about 3 percent of the nonseminomatous germ cell tumors. Most of the cases occur in combination with other cell types. At ultrasound, embryonal cell carcinomas are predominantly hypoechoic lesions with ill-defined margins and an inhomogeneous echotexture. Echogenic foci due to hemorrhage, calcification, or fibrosis are commonly seen. Twenty percent of embryonal cell carcinomas have cystic components. The tumor may invade into the tunica albuginea resulting in contour distortion of the testis [Fig. 4]. Yolk sac tumor Yolk sac tumors also known as endodermal sinus tumors account for 80% of childhood testicular tumors, with most cases occurring before the age of 2 years. Alpha-fetoprotein is normally elevated in greater than 90% of patients with yolk sac tumor (Woodward et al., 2002, as cited in Ulbright et al., 1999). In its pure form, yolk sac tumor is rare in adults; however yolk sac elements are frequently seen in tumors with mixed histologic features in adults and thus indicate poor prognosis. The US appearance of yolk sac tumor is usually nonspecific and consists of inhomogeneous mass that may contain echogenic foci secondary to hemorrhage. Choriocarcinoma --- Choriocarcinoma is a highly malignant testicular tumor that usually develops in the 2nd and 3rd decades of life. Pure choriocarcinomas are rare and represent only less than 1 percent of all testicular tumors. Choriocarcinomas are composed of both cytotrophoblasts and syncytiotrophoblasts, with the latter responsible for the clinical elevation of human chorionic gonadotrophic hormone levels. As microscopic vascular invasion is common in choriocarcinoma, hematogeneous metastasis, especially to the lungs is common. Many choriocarcinomas show extensive hemorrhagic necrosis in the central portion of the tumor; this appears as mixed cystic and solid components at ultrasound. Teratoma Although teratoma is the second most common testicular tumor in children, it affects all age groups. Mature teratoma in children is often benign, but teratoma in adults, regardless of age, should be considered malignant. Teratomas are composed of all three germ cell layers, i.e. endoderm, mesoderm and ectoderm. At ultrasound, teratomas generally form well-circumscribed complex masses. Echogenic foci representing calcification, cartilage, immature bone and fibrosis are commonly seen [Fig. 5]. Cysts are also a common feature and depending on the contents of