Objectives: Secundum atrial septal defects (ASD) are common congenital heart anomaly with left to right shunt. Pulmonary arterial hypertension (PAH) can develop because of the shunt flow. Chronic exposure of increased blood flow may lead to histological changes in pulmonary artery vasculature and result in development of PAH. The clinical course of patient undergoing transcatheter closure of ASD with PAH is not well studied. In this study, we investigated the midterm outcomes in transcatheter closure of ASD with PAH. Methods: From May 2003 to December 2007, 359 patients underwent transcatheter closure of ASD using Amplatzer Septal Occluder (ASO, Golden Valley, MN) in our institute. Among them, 43 patients (12%) had pulmonary hypertension at the time of transcatheter closure of ASD. Pulmonary hypertension was classified as mild (40-49mmHg), moderate (50-59mmHg), severe (above 60mmHg) according to pulmonary artery pressure measured by cardiac catheterization. At follow-up, PAH was measured as right ventricular pressure calculated by echocardiography. We reviewed the post procedure echocardiography data, demographic data, clinical characteristics, course of the PAH and complications in these subjects. Results: There were no mortalities in all included patients. In mild PAH group (n=25), eighteen patients (72%) had normalization immediately after the transcatheter closure of ASD, and the remaining 7 patients had normalization during follow-up period. In moderate PAH group (n=10), eight patients (80%) showed normal pulmonary artery pressure after closure of ASD, and 2 patients normalized within 1 month. In severe PAH group (n=8), there were only 1 patient (12.5%) who showed immediate normalization after transcatheter closure. Normal pulmonary artery pressure was shown in six patients who had remaining PAH after the procedure during follow-up period. One patient with severe PAH was treated with selective pulmonary artery vasodilator (Bosentan). Only mild PAH remained at last follow-up. No significant complications related to ASD occlusion and PAH were shown. Conclusion: Transcatheter closure of ASD with PAH may be a useful tool in treatment of patients of ASD with PAH. Because of the ASD shunt flow amount, PAH may be more aggravate and make irreversible changes in pulmonary artery vasculature. Therefore, in selected patients, transcatheter closure of ASD with PAH can be a useful method of treatment, for closing the defect as well as the improvement of PAH. Further study with more patients and long term follow-up will be needed.