It has been reported that cardiac function can be improved by implanting a DDD pacemaker (PM) and setting a short atrioventricular (AV) delay in patients with impaired cardiac function. A previous report found that the critical AV delay that induces diastolic mitral regurgitation (MR) may represent the upper limit of the optimal AV delay. The optimal AV delay can be predicted by a simple method: slightly prolonged AV delay minus the interval between the end of the atrial kick and complete closure of the mitral valve (duration of diastolic MR) at the AV delay setting. The patient was a 84-year-old man with an old myocardial infarction. He had repeated admissions to hospital for congestive heart failure. ECG showed prolongation of the PQ interval (0.28 s) and complete left bundle branch block. Cardiac function was improved by AV sequential pacing when the AV delay was set at 120ms. After DDD-PM implantation, the cardiothoracic ratio decreased from 57 to 45% and cardiac function was improved from New York Heart Association class III to I. The AV delay was optimized during follow-up. Four years after PM implantation, the patient was in good condition without further hospital admission.