Background: Transcatheter closure of atrial septal defect (ASD) using Amplatzer septal occluder (ASO) has been reported as an effective therapeutic option. However, long-term outcomes after this procedure remain to be clearly established, especially in Eastern countries. Therefore, we sought to evaluate intermediate-term prognosis and complications after transcatheter closure of ASD using ASO. Methods: ASD closure using ASO was performed in 176 adult patients from August 2002 to April 2009 in Asan Medical Center. There was no mortality related with the procedure. Of 176 patients, device implantation was successful in 158 (89.8%) patients (age 16 to 76 years, median 41 years, 118 females). In these patients, maximal ASD diameters measured by transesophageal echocardiography were ranging from 2 to 31 mm (median: 18 mm) and aspirin 100 to 200 mg per day was prescribed at least for 6 months after procedure in all patients. The median clinical follow-up duration after procedure was 9.1 months (inter-quartile range: 3.9 – 19.1 months). Results: During the follow-up period, there were no deaths, cardiac perforations, device embolizations, thromboembolisms, or infective endocarditis. Atrial fibrillations were developed in 6 (3.8%) patients 2 days to 10.8 months (median 10 days) after device implantation. Of those 6 patients, atrial fibrillation was spontaneously converted into sinus rhythm in 5 patients. Headache was complained in 9 (5.7%) patients 13 days to 8.1 months (median 2 months) after device implantation. Chest discomfort without documented tachyarrhythmia developed in 25 (15.8%) patients 10 days to 49 months (median 3 months) after procedure. All these symptoms were spontaneously resolved or tolerable with adequate medications. Conclusion: ASD closure using ASO seems to be a safe procedure with excellent intermediate-term prognosis except several minor complications and symptoms.