A pacemaker-bearing patient with left-sided breast cancer was treated with adjuvant external beam radiation therapy to the intact breast. She was treated via tangential fields and a single anterior supraclavicular field using 6-MV x-rays. The pacemaker, originally in the treatment field, was removed and a new one placed 4 cm outside the radiation field prior to treatment. Silicon diode chamber Keithley-Farmer type 0.6 cc ionization chamber, and lithium fluoride (LiF) (TLD) chips were used to measure, in vivo, the dose to the pacemaker. From all the fields treated, total dose to the pacemaker was 164 cGy by diode measurements, 182 cGy by ionization chamber measurements, and 171 cGy by TLD measurements. The pacemaker functioned normally throughout the course of treatment.