Carcinoma of the mastoid is infrequent in the general population.1-5 In the selected population of radium-dial painters and patients treated three or four decades ago with radium water or radium-like salts the incidence is markedly increased. During recent years over 300 patients with radium poisoning were followed at the University of Chicago Hospitals and Argonne National Laboratories.* Eight of these patients developed carcinoma of the mastoid, and six have died because of their tumor.6 At necropsy the temporal bones of two of these patients were removed, placed in 10% formalin, and submitted to the Temporal Bone Bank for processing. The purpose of this paper is to discuss the problems of radium poisoning and present the histopathological findings in the temporal bone specimens of these patients. Carcinoma of the mastoid as recorded in the literature presents with symptoms of aural discharge, pain in the ear which may radiate to