The incidence of cardiac arrhythmias has been studied in 50 unselected surgical patients with preexisting ECG abnormailties preoperatively, by means of continuous monitoring, utilixingan eIectrocardioscope, Burdick CS-515 Monitor, and was recorded by EK-4S ECG, at the Kyung Hee University Hospital, Seoul, Korea. The results are as follows: 1) Among 50 patients, 32 cases (64%) developed a variety of arrhythmias during general anesthesia and operation. It also showed arrhythmias excluding sinus tachycardia in 14 cases. The 28 per cent incidence of arrhythmias was rather high compared with the patient groups without preexisting heart disease or arrhythmias, such as in our previous report in 1972 of 20%, Dodds 19, 9% and Vaniks 16. 3%. 2) The most common arrhythmia seen was sinus tachycardia (26 cases, 65% of the total arrhythmias) and the next one was premature ventrieular contraction. 3) Nine cases (22. 5%) were considered serious in nature, such as premature ventricular contractions. 4) There was an increased incidence of new arrhythmias in the patients in poor general condition. 5) The vast majority of arrhythmias seen during anesthesia, except for 3 cases did not require drug treatment. It is usual to eliminate the cause of the arrhythmia by lowering anesthetic depth, eliminating excess carbon dioxide by hyperventilation and proper oxygenation of the lungs. 6) Constant electrocardiographic monitoring of patients under anesthesia has proven to be valuable adjunct to the administration of clinical anesthesia.