Permanent cardiac pacemakers are implanted so that the electrodes in contact with the heart are connected to a power source located elsewhere on the chest. This battery pack is usually placed in a subcutaneous pouch which overlies the right or left pectoral muscles. In this position it obscures the lung underlying the area when it is examined on a posteroanterior chest roentgenogram. Recently, an 80-year-old man who had previously had an artificial pacemaker implanted for complete heart block was found to have a malignant pulmonary tumor in this area (Fig. 1). The decision to use radiation as treatment for the tumor was complicated by the uncertain effect the radiation would have on the performance of the pacemaker. Pacemaker Radiation In order to determine the effect of radiation on similar pacemakers, two separate units were subjected to direct irradiation from a cobalt-60 teletherapy source. A ventricular demand type pacemaker2 was given a total dose of 10,000 rads, and a P-wave synchronous pacemaker3 ...