We developed a score predictive of survival following out-of-hospital cardiac arrest from an analysis of factors associated with 611 cases. The score is calculated from four pieces of information readily obtainable by emergency personnel directly at the scene. The four items are as follow: A, arrest witnessed; C, cardiac rhythm; L, lay bystander cardiopulmonary resuscitation (CPR); S, speed (response time of paramedic unit). Among 22 patients with favorable findings on all four predictive variables (witnessed arrest, ventricular fibrillation, bystander CPR, paramedic response time less than four minutes), 15 (70%) were discharged alive. The ACLS score for this group of patients was 70%. Among 97 patients with the most unfavorable findings (whose ACLS score was 0), one (1%) was discharged. We believe the score can provide emergency personnel with a realistic appraisal of the likelihood of successful resuscitation.