A 25-year-old male was presented in October 2004 with a monthly history of a gradually enlarging, painless, left cervical mass. He had no history of prior thyroid disorder or other disease. Clinically the mass was hard in palpation, firmly attached to the surrounding tissues and was located at the anatomic area of the carotide triangle. Thorough physical examination revealed only a marginal hepatomegaly and splenomegaly. Routine blood tests, biochemical markers and thyroid function tests – including triiodothyronine (T3), thyroid stimulating hormone (TSH), thyreoglobulin and anti-thyroid antibodies- were within normal limits, with the exception of a moderately elevated erythrocyte sedimentation rate (ESR = 44). Subsequent cervical and thoracic CT scan revealed a multilobular mass (4,3 × 4,7 × 7 cm) probably arising from the left thyroid lobe, with infiltrating features and heterogeneous density with regions of central necrosis and hemorrhage. The mass submerged into the anterior-posterior mediastinum, in proximity with the great vessels of the heart, dislocating the left common carotid artery and the left vagus nerve without infiltrating them. Significant mediastinal lymphadenopathy was also noted, whereas ultrasound of the abdomen excluded liver involvement. Differential diagnosis included thyroid carcinoma, lymphoma, thymoma, malignant congenital branchiac cyst or cystic hygroma and germ cell tumours of the upper mediastinum. Fine-needle aspiration biopsy of the cervical mass was performed and the cytological findings were consistent with papillary thyroid carcinoma with anaplastic features. Based on these findings, the patient was referred for surgical removal of the lesion. At surgery, a large mass, measuring 9,5 × 6,3 × 4,5 cm was found behind the left srenocleidomastoid muscle, located lateral to the left carotid artery/jugular vein and was not firmly adhered to the left thyroid lobe. The mass was easily separated from the surrounded tissues and was removed. Based on the results of preoperative fine needle aspiration biopsy, a total thyroidectomy was performed at the same time. Pathology examination of the mass revealed extensive infiltration by large or giant malignant cells with morphological features consistent with syncytiotrophoblasts within necrotic and hemorrhagic elements. Positive immunohistochemical staining with β-subunit of human chorionic gonadotrophin (β-hCG) suggested the diagnosis of choriocarcinoma of the upper mediastinum. The pre-operation serum concentration of β-hCG was >100.000 mIU/ml, whereas the immediate post-operation levels declined to 17.300 mIU/ml. Alpha- fetoprotein (a-FP) and carcinoembryonic antigen (CEA) levels were within normal limits, while lactate dehydrogenase (LDH) level was two-fold higher than the normal upper limit. Based on these data, scrotal ultrasound examination was performed, disclosing a small multilobular mass measuring 2,1 cm in greatest diameter-not evident at previously performed physical examination- located on the upper pole of the left testicle with echomorphological and hemodynamic characteristics consistent with seminomatous tumour. Subsequently, the patient underwent left radical inguinal orchiectomy with high ligation of the left spermatic cord and implantation of synthetic testicular prothesis. Histological diagnosis of the testicular tumour revealed almost typical seminoma (1,8 × 1,2 × 1 cm) consisting of large clear-cytoplasm cells with hypodense nucleus and a few atypical mitosis, without any signs of infiltration of rete testis or the spermatic cord. Complete inhibition of spermatogenesis and hyperplastic reaction of Leydig cells were also observed. Although immunohistochemical staining for β-hCG was positive in a few cells, their morphological characteristics did not meet the diagnostic criteria for syncytiotrophoblasts. On postoperative day six (6), the patient developed slightly painful cervical mass at the anatomic site of the first surgical intervention and β-hCG levels started rising up again (β-hCG = 29.850 mIU/ml). Complete pre-therapeutic staging was immediately performed, including negative CT scan of the brain and negative bone scan, whereas CT scan of the thorax and the abdomen disclosed multiple round metastatic nodules of various size (0,1 – 2 cm) in both lungs and marginally enlarged iliac and para-aortic lymph nodes without liver or other parenchymal organ involvement. In November 2004, one month after his initial admission to the hospital, the patient received 1st line chemotherapy for high-risk germ cell tumour with the BEP regimen (Bleomycin 30 mg: d1-d8-d15, Etoposide 100 mg/m2: d1-d5 and Cisplatin 20 mg/m2: d1-d5 in 21-day cycles). Pre-chemotherapy levels of β-hCG were 93.400 mIU/ml. The patient completed 4 cycles of therapy without experiencing remarkable toxicity (Neutropenia grade I-II according to the NCI-CTC criteria) and is currently (October 2006) asymptomatic, with ongoing complete clinical and biochemical remission according to the RECIST criteria (No evidence of tumour mass, regression of all enlarged lymph nodes, necrotic post-chemotherapy elements in the remaining lung nodules confirmed by CT-guided fine-needle aspiration biopsy and PET scan and consecutively normal levels of β-hCG). A schematic presentation of the whole diagnosis and treatment course including β-hCG titer and chest Xray findings is illustrated in Fig.