A 40-year old man admitted to the nuclear medicine department to undergo PET/CT for restaging of known testicular germ cell tumor. The patient had a history of orchiectomy for left testis and left nephrectomy with retroperitoneal lymph node dissection. Histopathology revealed testicular germ cell tumor. During the follow-up period, there is a rapidly expanding mass in posterior mediastinum. PET reveals increased FDG activity in the azygos vein. Contrast-enhanced CT of the thorax showed distended azygos vein with heterogeneously enhancing thrombus and peripheral fat stranding. Findings are consistent with azygos vein tumor thrombosis related to testicular germ cell tumor.