A 65-year-old man with a history of coronary artery bypass grafting was admitted because of severe heart failure. Echocardiography showed diffuse severe hypokinesis of the left ventricle (ejection fraction 25%) and severe mitral regurgitation caused by tethering of the leaflet secondary to left ventricular (LV) dilation. He underwent mitral valve annuloplasty and LV papillary muscle imbrication, but postoperative sustained ventricular tachycardia developed and echocardiography showed ventricular dyssynchrony with a long septal-to-posterior wall motion delay (>130 ms). Cardiac resynchronization therapy (CRT) was performed using a biventricular pacing system with an implantable cardioverter defibrillator, but biventricular pacing prolonged the QRS duration from 130 to 160 ms, so (11)C-acetate positron emission tomography was performed to evaluate the CRT. During biventricular pacing, myocardial oxidative consumption decreased by 15% and cardiac efficiency increased by 33%. The plasma brain natriuretic peptide level, which was 9,500 pg/ml preoperatively, decreased to 173 pg/ml just before discharge from hospital.