A 49-year-old woman developed complete atrioventricular (A-V) block 16 years after radiotherapy for breast cancer. The patient had received 60 Gy of cobalt 60 irradiation for left breast cancer arising 3 years after contracting right breast cancer. It took one year before complete A-V block was fixed, during which two-degree A-V block, atrial fibrillation, and sinus rhythm were predominant. She had myocardial insertion of pacemaker electrode. Fibrosis and hyaloid degeneration of the pericardium and thickened sclerosis of the epicardium seemed to be radiation-induced. There was no evidence of recurrence of breast cancer. (Namekawa, K.).