Germ-cell testicular cancer (GCT) is the most common solid cancer in younger men between puberty and 35 years of age, with a recorded increase in incidence in the last several decades. There are two histologic types of GCTs: seminoma and nonseminomatous germ-cell tumours which include embryonic carcinoma, choriocarcinoma, yolk sac tumour and teratoma. Any kind of seminoma - nonseminomatous tumour combination can be diagnosed in the same patient. While the majority of germ-cell tumours are only locally aggressive, a certain number has the potential for malignant transformation into tumours with somatic tissue elements. Development of somatic-type malignancy is rare and has been identified in approximately 3-6 % of patients. Histologically speaking, these are most commonly adenocarcinoma and various types of sarcomas. Although there is no histological resemblance, the presence of isochromosome 12p is evidence of their GCT origin. Somatic-type malignancy can be found in tissue samples after orchiectomy, and also in retroperitoneal lymph nodes after chemotherapy considering their lack of response to platinum-based chemotherapy. Clinically, presence of malignant transformation cay be suspected when there is evidence of tumour progression despite decreasing tumour marker levels. The purpose of this study was to describe testicular germ-cell tumours with malignant somatic transformation and their epidemiological, clinical and morphological features. Every clinician treating germ-cell tumours should be aware of the existence of this entity in tumour progression cases despite implemented traditional chemotherapy. Patients with testicular germ-cell tumours with malignant somatic transformation have a poor prognosis especially if the disease cannot be surgically treated. due to its lower sensitivity to chemotherapy and thus poorer prognosis.