This is a case of pure yolk sac tumor (YST) of the mediastinum showing well-developed Schiller-Duval bodies. Immunohistochemically, YSTs are positive for AFP (50% to 100% cases; patchy), SALL4, glypican-3, and PLAP (patchy; weaker than in seminoma). They are negative for OCT3/4, CD117, and beta-hCG. The prognosis of pure yolk sac tumor in children younger than 15 years is good if the tumor is completely resected. The 5-yr survival rate is greater than 80%. If incompletely resected, it drops to around 40%. In patients older than 15 years, the 5-yr survival rate is between 45% and 55%.