Somatic malignant transformation in mature cystic teratoma is a rare phenomenon of a malignancy of differentiated tissue structures of any stem line. The authors present a case of a 38-year-old female with mature cystic teratoma of both ovaries and with mucinous adenocarcinoma arising from endodermal germ line in the right ovary, showing immunohistochemical features of non-intestinal differentiation. At the time of diagnosis the tumour metastasized to the pelvic and retroperitoneal lymph nodes. The patient was treated with three lines of chemotherapy and died after 15 months with signs of massive progression into the retroperitoneal, mediastinal and cervical lymph nodes, retroperitoneum, duodenal wall and peritoneal cavity. Somatic malignant transformation in mature cystic teratoma is associated with poor prognosis. The most important prognostic factor is tumour stage at the time of diagnosis.