The development of somatic-type malignancy in a non-seminomatous germ cell tumor is a rare complication that occurs with greater frequency in mediastinum as compared to the gonads. The malignancy may be adenocarcinoma, neuroendocrine tumors, rhabdomyosarcoma, malignant peripheral nerve sheath tumor, or angiosarcoma among others. This patient developed mucinous (colloid) carcinoma in the background of a mature teratoma of the mediastinum. The resected specimen shows large cystic areas filled with jelly-like material resulting from the pools of extravasated mucin. Images 15-23 are from the same case.