This is an example of somatic-type malignancy (rhabdomyosarcoma) arising in a non-seminomatous germ cell tumor (teratoma) of the mediastinum. The patient was a 25 y/o male who presented with shortness of breath. A chest CT showed a large heterogenous mass in the anterior mediastinum. Serum alpha-fetoprotein was markedly elevated. Serum beta-hCG was within reference range. The resection specimen measured 10 cm in largest dimension and showed a yellow-tan solid and cystic tumor with pearly white islands of cartilage as well as firm, fleshy foci. The image shows nodules of hyaline cartilage and bone surrounded by a spindle cell proliferation. Rhabdomyoblasts with abundant eosinophilic cytoplasm were present elsewhere in the specimen. The spindle cells as well as the rhabdomyoblasts were strongly positive for desmin and myogenin and focally positive for S-100 protein.