A 78-year-old woman with chest pain and a history of pacemaker implantation for arrhythmia underwent myocardial perfusion imaging by means of N-NH3 cardiac PET/CT. N-NH3 cardiac PET showed no signs of ischemia or infarction. Incidentally, a nodule with increased N-NH3 activity was observed in the right breast. Histopathologic examination revealed invasive ductal carcinoma.