Observation of hemodynamic findings, E.K.G. findings and correlation between them were performed on 36 cases of atrial septal defect (secundum type) which were confirmed by cardisc catheterization and operation at the Department of Pediatrics and Chest Surgery, Seoul National University Hospital, during the past 13 years period from 1963 to 1976. The results were as follows; 1. On the E.K.G., 28 cases (77.8%) showed right axis deviation and 27 cases(75.0%) showed rSR?pattern on the right precordial lead. (V3R, V1) 2. Three cases showed increase in oxygen content over 0.9 vol. percent at the right ventricle over the right atrium 3. 20 cases (65.5%), among the total 31 cases, showed pressuere gradient of some degree between the systolic pressure of right ventricle and pulmonary artery, because of functional stenosis of pulmonary artery. In one case these was pressure gradient of 22 mmHg. 4.In 28 cases among the total 32 cases, showed pulmonary arterial pressure of less than 40mmHg, and 4 cases (12.4%) was more than 40mmHg. 5. All cases except one showed Qp/Qs of more than 2:1, and Qp/Qs of 2?:1 was the most common and 13 cases (44.8%), among the total 29 cases, belonged to this group. 6. Correlations between E.K.G. findings and right ventricular systolic pressure, were observed. In the group, with the right ventricular pressure over 40 mmHg, frontal QRS axis was shifted more to the right, a decrease in the incidence of rSR?pattern on the right precordial lead and an increase in the incidece of R/S ratio of greater than one in V1 were observed. It was also apparent that the average height of R wave in V1 increased with increasing pressure.