A series of 132 consecutive cases of secundum type atrial septal defect operated from 1963 to 1980 in the department of Thoracic Surgery of Seoul National University Hospital is analysed. The ages of the patients ranged from 12 months to 57 years and the ratio of men to women was 1 to 1.4. Clinical analysis revealed 33 patients [26%] were asymptomatic and 15 patients [12%]had congestive heart failure. Hemodynamic analysis revealed that severe pulmonary hypertension which is the most serious risk factor is ASD developed in 10 patients [9%] and pulmonary artery pressure significantly increased after the age of 20 years. There were 10 patients with sinus venosus defect and 7 patients with partial anomalous pulmonary venous return. The operative mortality was 2.2%.[KTCS 1981;3:241-246]