The malignant germinal tumors found in the testis (seminoma, teratocarcinoma, embryonal carcinoma, choriocarcinoma, and mixtures of these) can originate in the anterior mediastinum. An occult testicular tumor must be sought, but primary mediastinal germinal tumors constitute a true entity. The details of 24 such patients treated at Walter Reed General Hospital are presented. Of 19 patients who died, 2 were women; postmortem examination of the testes of 13 men failed to reveal a primary tumor. Five patients are alive and well with no testicular abnormality. It is important to distinguish the histopathologic types. Seminomas are readily controlled with modest doses of irradiation. Embryonal carcinomas are also radiocurable with somewhat higher doses; 2 long-term disease-free survivors are reported. Teratocarcinomas are highly malignant and require higher doses for control. Choriocarcinomas progress so rapidly that chemotherapy is necessary.