X-rays in different planes, roentgenisation, tomograms and not so often the clinical symptoms today are still responsible for the diagnosis. Mediastinoscopy and lateral mediastinoscopy very often ensure the diagnosis and dignity of the tumor. From 1956 to 1976 948 patients at our clinic were operated because of a mediastinal tumor. In 67 or 7% it was a thymic tumor. With thymic hyperplasia, thymoms and cysts of the thymic gland the survival time after operation was hardly reduced. The prognosis of the malign tumors of the thymic gland was bad with or without ray-treatment. By thymectomia 43% of the patients with myasthenia showed a good recovery.