Thirteen infants were found to have atrial septal aneurysms (ASAs) on two-dimensional (2D) echocardiography and their characteristics and natural course have been studied by 2D Doppler echocardiography. Atrial septal aneurysm was accompanied by atrial septal defect (ASD) or other cardiac malformations in 12 of 13 patients, and as an isolated anomaly in one patient. Spontaneous closure of ASD by aneurysmal formation of the interatrial septum was observed in 10 patients. The aneurysms of all 13 patients showed a moderate reduction in size, 14 to 25 months (mean 18 months), after these aneurysms had been first diagnosed and 10 subsequently disappeared. It is likely that ASA occurring in infancy will develop in patients with spontaneous closure of an ASD, and that such an aneurysm has a tendency to regress as the patient grows up.