Five hundred consecutive sudden deaths ascribed to coronary arteriosclerotic heart disease after complete autopsy were studied, with special attention to coronary vessels and myocardium. In 184 cases, new disease (recent thrombosis, acute myocardial infarct, or both) explained the sudden catastrophe. The remaining cases presented only chronic degenerative coronary disease, frequently associated with healed infarcts. The cardiac mechanisms responsible for sudden death in coronary disease must of necessity differ in these two groups with such disparate morbid anatomy. The presence of an intact myocardium in many of these patients suggests that the potentiality for good cardiac function exists if the heart beat can be restored. Such positive measures as intermittent rhythmic manual compression ("cardiac massage"), electrical defibrillation, or both in association with artificial respiration are indicated when the physician is present at deaths occurring under these circumstances.