While the sole purpose of this paper is to show through the detailed report of individual cases that radium may be used as an effectual remedy for many primary mediastinal new growths, an incurable habit of introduction seems to make indispensable a few preliminary statements dealing broadly with the clinical and pathologic aspects of the diseases under consideration. Malignant new growths are much commoner than benign in the mediastinum. Metastatic lymphosarcoma, Hodgkin's disease, carcinoma and sarcoma are very frequent, while primary mediastinal malignancy is rare. In these secondary cases, however, the primary lesion almost invariably dominates the clinical picture, from the standpoints both of diagnosis and of treatment. A roentgenographic examination of the chest should be made before the treatment, operative or otherwise, of any malignant growth is undertaken, particularly in cases of tumors of the thyroid, ovary, breast and testicle. The very interesting data which we have accumulated as