Teratoma is a germ cell tumor, which consists of derivatives of three germ layers and has various malignant potentials – from benign mature forms to immature embryonic forms with a somatic type of malignancy. Even the mature type of teratoma is biologically unpredictable providing for an ability to grow locally with invasion. Necessity for complete removal of tumors, wherever they are located, is explained by the fact that there is a risk of a possible development of growing teratoma syndrome or transformation into malignant tumors. This article describes the clinical case of growing teratoma syndrome of mediastinal localization in a teenager with Klinefelter syndrome, also as an example of the multidisciplinary approach of pediatric oncologists, surgeons, radiotherapists and pathomorphologists in decision-making for optimal treatment. The patients' parents gave their consent to the use of their child's data, including photographs, for research purposes and in publications.

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