• Cardiac arrest is the sudden cessation of demonstrable cardiac activity unexplained by gross hemorrhage, shock, or asphyxia. In the decade 1945-1954 it was taken as the probable cause of 50 out of a total of 84 operating room deaths that occurred during 103,777 surgical procedures. In this decade the proportion of older patients was higher than in the preceding two decades, and the highest percentage of cases of cardiac arrest occurred in the 70-79 year age group. The incidence was 30 times as high in patients in admittedly poor physical condition as in patients in good condition; it was also higher in patients with heart disease. Additional contributing factors were the deepening of anesthesia, hypoxia, reflex phenomena, and improper choice or management of anesthesia. Emergency thoracotomy and manual systole were carried out in 58 cases of cardiac arrest, with 26 recoveries. During the last 10 years the recovery rate in cardiac arrest was 37%, and during the last 5 years it was 50%.