As a major cause for postoperative hypoxia, the importance of increased physiological shunting is increasingly emphasized. This study is a review and analysis of postoperative physiological shunting following open heart surgery with the aid of extracorporeal circulation. Sixteen patients were selected from among 21 patients who underwent elective open heart surgery at the Department of Thoracic and Cardiovascular Surgery, Kyungpook National University, School of Medicine, from December, 1975 to September, 1977. The results were as follows: 1. The degree of postoperative physiological shunt was progressively increased from 18.8% mean value one hour after surgery to 22.7% mean value, reaching a peak on the second postoperative days. 2. For up to one week, large physiological shunt[15%] was persisted in one patient. 3. Comparing long[more than 90 minutes] with short[less than 90 minutes] perfusion time group using pump oxygenator, it was found that the physiological shunt increased about 3% in the long as compared with the short perfusion time group. 4. The mean blood pressure was 70-80 mmHg without a remarkable causal relationship between physiological shunt and mean blood pressure. 5. On elevated PaO2[>200 torr], the physiological shunt was decreased less than 20% of cardiac output, but on diminished PaO2[102 torr] after two days, it was 22.7% of cardiac output. From above results, a contrary causal relationship between PaOz and physiological shunt was obtained. Co Reviewing chest X-rays postperfusion, it was demonstrated that no remarkable causal relationship between roentgen-ray evidence and physiological shunt could be obtained.[KTCS 1977;2:274-280]