From Jun. 1980 to Mar. 1984, 33 cases of atrial septal defect, secundum type, operated at the department of Thoracic and Cardiovascular Surgery, School of Medicine, Keimyung University, were analyzed retrospectively. They were 24.3% of all congenital heart diseases operated in the same period. Among the 33 cases, 17 cases were males and 16 cases were females. Their ages were ranged from 4 to 42 years and cases of over 15 years of age were 60.6%. Main symptoms at admission were exertional dyspnea [69.7%], palpitation [63.6%] and frequent upper respiratory infections [51.5%], but 2 cases [6.1%] were asymptomatic. All 33 cases were operated under the direct vision with cardiopulmonary bypass. All cases were secundum type defect of atrial septal defect and single defect were in 29 cases [90.6%], and oval type defect were in 31 cases [96.9%]. In 7 cases [21.9%], other lesions of cardiovascular system were associated, and the most common lesion was pulmonic valvular stenosis [4 cases; 12.5%]. The defects were closed directly in 27 cases and in 6 cases with Dacron patch. Postoperative complications were occurred in 8 cases [24.2%], and they were pleural effusion, congestive heart failure, and alopecia mainly. One case died due to air embolism postoperatively and operative mortality was 3.1%.