The effects of ionizing radiation on the ears of 100 patients were studied in the course of treatment of malignant head and neck tumors by teleradiation using cobalt-60. The early changes consisted of "radiation otitis media" and a transient vasculitis of the vessels of the inner ear, resulting in hearing loss, tinnitus, and temporary recruitment. While no permanent changes could be detected microscopically shortly after the completion of radiation in the cochlea or labyrinth, late changes sometimes occurred in the temporal bone as a result of an obliterating endarteritis. These late changes represented an entirely different clinical entity. Results of animal experiments, as the review of the literature reveals, are in agreement with the clinical findings observed in man exposed to ionizing radiation in therapeutic doses.