his study was performed to follow up the natural course and size change of isolated secundum atrial septal defect (ASD). Among the newly diagnosed with secundum ASD at the division of pediatric cardiology in Chonnam National University Hospital from January 2001 to December 2003, 60 patients with isolated secundum ASD were checked by the serial echocardiographic evaluation to measure the size change. The age at diagnosis was 6.9 months. Forty three percent of ASD showed spontaneous closure, and 23% decreased to a diameter of 4 mm. Of ASD with diameter of <6 mm at diagnosis, 57% showed spontaneous closure, 31% regressed to a diameter of 4 mm, and none required surgical closure. Of ASD with a diameter of >10 mm at diagnosis, none closed spontaneously, and all of them required surgical or device closure. The possibility of spontaneous closure of isolated secundum ASD was higer in group of patients who have smaller defect and who were younger.