A score predictive of survival following out-of-hospital cardiac arrest was developed from an analysis of factors associated with 611 cases. The score is calculated from 4 pieces of information readily obtainable by emergency personnel directly at the scence. The 4 times are as follows: arrest witnessed [A]; cardiac rhythm [C]; lay bystander cardiopulmonary resuscitation (CPR) [L]; speed (response time of paramedic unit) [S]. Among 22 patients with favorable findings on all 4 predictive variables (witnessed arrest, ventricular fibrillation, bystander CPR, paramedic response time < 4 min), 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), 1 (1%) was discharged. The score can provide emergency personnel with a realistic appraisal of the likelihood of successful resuscitation.