Angiography of the internal mammary artery was performed in 30 patients with suspected tumors in the anterior mediastinum. In 3 patients, thymic venography was also performed. In 13 lymphoepithelial thymomas, a smooth outline with the arteries penetrating the periphery of the tumor was demonstrated. Frequently, neovascularity but never encasement of large arteries or arteriovenous (AV) shunting was exhibited. In the lymphoepithelial thymomas of the cell-rich type, the accumulation of contrast medium and neovascularity were pronounced. This type of thymoma may possibly be locally more invasive than others. In carcinomas, the arteries penetrated the tumor and AV shunting was common. Arteriography was superior to venography in the few patients in whom both examinations were performed. Angiographically, it should be possible to differentiate thymomas from carcinomas.