An 80-year-old woman diagnosed with sarcoidosis at 74 years of age had been continuously on oral low-dose corticosteroids for treatment. Four years later, she developed complete right bundle branch block on ECG, and one year later, ECG changes as a first-degree atrioventricular block were observed. However, she remained asymptomatic. After one year, she developed acute onset of cardiac failure. ¹⁸F-fluorodeoxyglucose positron emission tomography (FDG-PET) revealed strong uptake in the bilateral hilar and mediastinal lymph nodes and in the heart of the anteroseptal and lateral walls and the right ventricle. These were suspected to be cardiac lesions associated with sarcoidosis. Following intensive steroid therapy, the patient exhibited improvement in terms of the atrioventricular block and FDG-PET findings. Physicians in the respiratory medicine field should be careful regarding the development of cardiac lesions among cases with sarcoidosis. In particular, serial changes on ECG findings might be important for suggesting the existence of cardiac involvements even though they have no symptoms or signs.