RESUMO
BACKGROUND: Germ cell tumors are tumors composed of tissues derived from more than one of the three germinal layers. They are more common in the testes and ovaries, but can present in many different regions in the midline, including the sacral region, retroperitoneum, mediastinum, and brain. Testicular germ cell tumors generally metastasize to the retroperitoneum, lungs, and brain; metastases to soft tissue are very rare. CASE PRESENTATION: Here we describe a case of a single soft-tissue mass in the thigh of a 27-year-old man, with histology showing areas of mature teratoma tissues derived from the ectodermal and mesodermal lineages, and areas of immature teratoma tissue composed of small undifferentiated cells, with primitive neuroectodermal differentiation foci forming neuroepithelial elements - thus classified as immature teratoma. The patient had no other clinical or radiological evidence of involvement, besides the lymph nodes. CONCLUSION: The case presented suggests a rare and unexpected primary immature teratoma of the thigh.