Thirty seven patients with ostium secundum atrial septal defect, operated from January, 1976 to September, 1984 at the Department of Thoracic and Cardiovascular Surgery, Kyungpook National University Hospital, were given clinical assessment. The following results were obtained. 1.Ostium secundum atrial septal defect was comprised of 18% of congenital heart disease. Their mean age was 15.7ulcorner.42. Sex ratio [male:female] was 1:1.1. 2.Most frequent clinical symptom was dyspnea on exertion occurred in 26 patients [76.5%]. Only one patient had no symptom [2.9%]. 3.Pre-operative EKG findings revealed RVH in 61.8%, ICRBBB in 29.4%, and RAD in 41.2%. 4.Mean value of systolic pulmonary arterial pressure in patients over 20 years old was 37.8ulcorner4.4mmHg and it was 28.1ulcorner10.2mmHg in patients under 20 years old, but the difference between two groups was not statistically significant. 5. In large defect group [>3cm in diameter], Qp/Qs was significantly increased than small defect group [<3cm in diameter], but systolic pulmonary arterial pressure and Rp/Rs were not different between two groups. 6. Overall operative mortality was 5.4%.