Even though the pathogenesis is still controversial, electrocardiographic changes after congenital open heart surgery depend on various etiologic factors. Author reviewed 261 cases of congenital open heart surgery patient experienced in the Dept. of Thoracic & Cardiovascular Surgery, Pusan National University Hospital, since July, 1981 to Sept., 1985. The results were summarized as followed: 1. The age distribution was from 3 to 29 years old with mean age 10.8 years. And the most frequent congenital heart defect we had done operation was VSD. 2. Preoperatively, the most frequent ECG finding was abnormal QRS complex and postoperatively the most frequent ECG change was arrhythmia. 3. The most frequent arrhythmia before operation was intraventricular conduction disturbance. 4. Right ventriculotomy incision produced the more frequent abnormal ECG changes postoperatively than right atriotomy or pulmonary arteriotomy. 5. According to the operative technique, outflow patch graft of TOF repair produced the highest frequency of ECG changes after operation, and in comparing simple and patch closure of VSD, the latter was higher frequency of ECG changes, in valvotomy and infundibulectomy of PS, the latter was higher too. 6. The common symptom and signs in abnormally ECG changed patients after operation were palpitation, dyspnea, congestive heart failure and murmur in this order., 7. The longer the time of CPB, the more number of abnormally ECG changed patients had been developed after operation. 8. The most of postoperative ECG changed patients recovered spontaneously or with only medication and were clinically insignificant. And the rest other minor group recovered with temporary pacing. Eight cases out of 261 [3.1%], these with abnormal QRS complex and arrhythmia couldn`t recover in spite of every effort and eventually succumbed.