Mature cystic teratoma is the most common type of mediastinal germ cell tumor. They can become quite large and often contain calcifications. The cut surface usually shows solid and cystic areas. Mucoid or sebaceous material may escape from the cysts and cause xanthogranulomatous inflammation as well as fibrous adhesions with the contiguous structures such as lungs. This specimen is from a 12-year-old girl who presented with left sided chest pain. A chest CT showed a mediastinal mass with calcifications. At surgery, a large encapsulated mass was encountered adherent to the left lung. The resection specimen was multicystic and the cysts were filled either with mucoid material or yellow-white cheesy material.