1. Clinical and physiological data in 32 children with atrial septal defect are presented. 2. The physical findings in the typical case are characterized by an upper left sternal border soft systolic murmur, apical diastolic murmur and widely split pulmonic closure. 3. The radiological findings consist of right ventricular and atrial enlargement with prominent main pulmonary artery and pulmonary vascular engorgement. 4. The electrocardiograms show right bundle branch block with right ventricular hypertrophy. 5. The physiological data demonstrate a marked increase in pulmonary blood flow with no significant diminution in systemic cardiac index and the absence of pulmonary vascular disease. 6. The aim of surgical repair of atrial septal defect is the abolition of the left to right shunt. 7. Candidates for operation are patients with large atrial shunts and significant symptomatology or marked cardiomegaly without markedly increased pulmonary vascular resistance. 8. Operative results in nine children are presented.