A 61 year old man who had been studied extensively died unexpectedly ("instantaneously") outside the hospital while wearing an electrocardiographic recorder. Death was caused by ventricular fibrillation, which was initiated by an early cycle ventricular premature contraction occurring in the vulnerable period of repolarization. Such early cycle ventricular premature contraction had been noted in recordings 4 years previously but had never been observed to encroach on the T wave until 5 minutes before death. In the intervening period, the patient had shown increasing evidence of myocardial ischemia and hypertrophy and congestive heart failure, which had been partly obscured by his concealment or denial of symptoms and refusal to change his pattern of activities. Autopsy revealed two old myocardial infarcts and pronounced left ventricular hypertrophy. There was advanced occlusive arteriosclerosis of the major coronary vessels with a recent thrombus in the right coronary artery.