Introduction: Atrial septal defect (ASD) is a commonly known congenital heart anomaly with left to right shunt, and pulmonary arterial hypertension (PAH) can develop over time. PAH is an important factor in determining the optimal mode of therapy, as well as the reason for increased morbidity and mortality. The object of this study was to investigate the midterm progress of patients with ASD and severe PAH after closure of the defect by evaluating the changes of pulmonary artery pressure and the clinical course. Method and Results: From March 2006 to February 2011, 442 adult patients underwent closure of ASD by both surgery and transcatheter device and of them, 19 (4%) patients were classified as severe PAH. Severe PAH was classified as systolic pulmonary artery (PA) pressure above 60 mmHg according to catheterization data before the closure of defect. Of the 19 patients, 11 (Group A) underwent transcatheter closure of ASD and the remaining 8 (Group B) patients underwent surgical correction. Patients with remaining PAH after closure of the defect was treated with selective pulmonary artery vasodilator(s). The patients were followed-up for at least 6 months, and normal pulmonary artery pressure was maintained in 7 (63%) patients in Group A and 6 (75%) in Group B during follow-up. Conclusion: Correction of ASD is safe and efficacious in adult patients with ASD and associated severe PAH, but patient selection must be meticulously made since it may be fatal in very elderly patients with serious comorbidities. Pulmonary vascular resistance and PA pressure prior to the treatment determines the clinical course of the patients, so precise evaluation is crucial in this group of patients. The new generation pulmonary vasodilators can help improve functional class of patients with severe PAH, and the treatment of ASD with PAH can be extended.