Of 133 persons with spontaneous cardiac arrest attended by paramedics within 10 minutes, 100 (75%) had ventricular fibrillation as the initial rhythm and 33 (25%) had extreme bradycardia or asystole. The latter group of arrhythmias was characterized by sinus arrest or severe sinus bradycardia (90%) and complete A-V block (10%). Junctional escape rhythm was also absent or markedly retarded. Despite cardiopulmonary resuscitation and the administration of epinephrine, atropine, isoproterenol, and sodium bicarbonate, recovery of the sinus and junctional tissues was infrequent. Ventricular fibrillation developed in 11 cases (33%). One patient lived 12 days, but all others were dead on arrival or died in the emergency room. Among the 13 coronary causes of death proved at autopsy, 10 (77%) were due to a fresh thrombus and seven (54%) to an occluded proximal right coronary artery, suggesting a causal relation to this type of arrest.