The medical literature portrays a bleak prognosis for out-of-hospital cardiac arrest cases presenting with asystole, idioventricular rhythms with pulselessness, or primary electromechanical dissociation. In view of evolving philosophies to waive resuscitation attempts in such cases, we sought to delineate the actual contribution toward overall survivorship that is provided by resuscitation efforts for patients who have these electrocardiographic presentations. Design: A prospective outcome study which analyzed all out-of-hospital cardiac arrest cases in a large city for a 2-yr period in terms of presenting electrocardiogram, age, sex, presence and status of witnesses, performance of bystander cardiopulmonary resuscitation, and survival to successful hospital discharge. Setting: A large urban municipality (population, two million) served by a single, centralized emergency medical services program. Patients: Excluding cases associated with trauma, drugs, airway obstruction, submersion or primary respiratory illness, 2,404 consecutive adult out-of-hospital cardiac arrest patients were studied. Interventions: Standard advanced cardiac life support. Measurements and Main Results: Although survival “rates” of patients with asystole, idio-ventricular rhythms with pulselessness, and electromechanical dissociation were low (1.6%, 4.7% and 6.9%, respectively), 22.2% of the 193 total survivors (confidence interval: +5.9%) initially presented with one of these electrocardiographic rhythms (14 asystole, 18 idioventricular rhythms with pulselessness, 10 electromechanical dissociation, plus one other). Conclusions: Despite poor survival “rates,” resuscitative efforts forpatients presenting with asystole, electromechanical dissociation, and idioventricular rhythms with pulselessness all contribute significantly toward a community's total survivorship from out-of-hospital cardiac arrest. Initial, aggressive attempts at resuscitation still should be emphasized in such patients. (Crit Care Med 1993; 21:1838–1843)