The preoperative pulmonary arterial pressure of 38 patients aged over 50 undergoing surgical closure of a secundum atrial septal defect was studied. They were divided into three groups according to systolic pulmonary arterial pressure (PAP): Group A (PAP<30mmHg, n=14), Group B (30≤PAP<50mmHg, n=16), and Group C (PAP≥50mmHg, n=8). The mean age of group C patients was older than that of group A patients. With higher PAP, the Pp/Ps, Rp/Rs and cardiothoracic ratios increased, atrial fibrillation and heart failure (NYHA≥2) were more frequent, and PaO₂ levels declined. There were no differences in left to right shunt ratio and Qp/Qs among the three groups. The PAP and Rp/Rs were under 70mmHg and 0.30, respectively in all patients. High pulmonary blood flow seems to be the cause of pulmonary hypertension in most elderly patients because PAP and Rp/Rs decreased after surgery in all groups. Findings of cardiomegaly and heart failure also improved after surgery. Surgical intervention is recommended even in elderly patients with a ASD.