A patient with cardiac pacing operated for cancer of the breast received subsequent radiotherapy, first with a linear accelerator, then with a betatron. The pacemaker was found deprogrammed as a sequel to irradiation although care was taken to keep it outside the irradiation field. After reprogramming the pacemaker changed a second time spontaneously the given parameters indicating permanent damage by the irradiation. In such cases the exchange of the pacemaker is recommended. Precautionary measures ranging from surveillance to possibilities for reprogramming and even reanimation or pacemaker replacement should be assured prior to irradiation of patients with cardiac pacing.