A 5-month-old girl presented with clinical symptoms of secundum type atrial septal defect (ASD 2), roentgenographic evidence of cardiac enlargement, and electrocardiographic signs of right ventricular hypertrophy. Cardiac catheterization showed an isolated atrial septal defect with pulmonary blood flow twice systemic flow, equal right and left atrial pressures, and elevated right ventricular pressure. The child was managed medically and the clinical, roentgenographic, and electrocardiographic abnormalities gradually disappeared. A second catheterization at the age of 5 years revealed no shunt, different atrial pressures, and normal right ventricular pressure. This is the seventh documented functional closure of isolated secundum type of atrial septal defect diagnosed after the neonatal period. It is recommended that optimal time be allowed for the possibility of spontaneous functional closure of atrial septal defect.