Evaluating the cardiopulmonary functions of 16 patients with atrial septal defect, the author obtained the following results. Cardiac Functions 1. Both the pulmonary arterial pressure and the ventricular pressure were normal or slightly elevated, but there were two cases showing a marked rise in pulmonary arterial pressure. The atrial pressure was normal. 2. The total pulmonary vascular resistance and the pulmonary arteriolar resistancc showed a slight increase, but there were some who showed a marked rise. 3. The ventricular work was normal in the left, but was increased in the right ventricle in all. In the case with marked pulmonary hypertension the ratio of ventricular work was in an inverse proportion and indicated a greater load on the right ventricle. 4. All these showing the cross-sectional area of atrial septal defect of over 5.0c㎡ were severe cases. The area of cross-section of septal defect was correlated to both shunt-flow and pulmonary blood-flow; and likewise there was a correlation between the pnlmonary arterial presture and the pulmonary vascular resistance. However, no definite relation existed between the pulmonary arterial pressure and pulmonary blood flow; and the area of cross-section of septal defect and the pulmonary vascular resistance play an important role in the cardiac functions of this disease. Pulmonary Functions 1. Minute ventilation volume in the cases with pulmonary hypertension as well as in cases showing increase in pulmonary blood-flow, shunt-flow, and in cross-sectional area of atrial septal defect tended to increase. 2. The maximal breathing capacity ratio, the vital capacity ratio, and various factors of circulatory functions were in inverse proportion to one another in the majority of cases. 3. In exercise test the increase iu the minute ventilation volume was slightly less than the increase of oxygen consumption. There was little difference between the oxygen equivalent upon exereise and that at rest.