Yolk sac tumor is usually seen in the mediastinum admixed with other germ cell tumors. Rarely, it may occur as a pure tumor in this location. The case presented here was a mediastinal teratoma with a large component of yolk sac tumor. The image shows glandular-alveolar pattern with a myxomatous background. The patient was a 15 y/o male who presented with dyspnea, dysphagia, and chest pain. Imaging studies revealed a large anterior mediastinal mass. Serum alpha-fetoprotein was elevated. The patient's condition showed rapid deterioration and after briefly responding to therapy, he died. Autopsy showed a large anterior mediastinal mass adherent to lungs and widespread metastases to liver and lymph nodes. The mass had solid and cystic areas containing keratinaceous debris and hair. The yolk sac component was strongly positive for AFP and negative for CD30.