Early diagnosis of atrial septal defects (ASDs) by means of two-dimensional echocardiography has facilitated studies on the natural course of this lesion. Among 12 Japanese infants in whom a diagnosis of opening of the atrial septum was made in the neonatal period, eight had spontaneous closure at age 2 to 16 months, as shown by two-dimensional echocardiography. To study the exact incidence and natural course of atrial openings in the neonatal period, another 102 consecutive infants from two neonatal care units were followed prospectively with the use of two-dimensional echocardiography. Atrial openings were evident in 24 infants (24%) within the first week of life, in 13 (13%) older than 1 week, in seven (7%) older than 1 month, in five (5%) older than 6 months, and in two (2%) older than 1 year. Twenty-eight of the 30 infants with spontaneous closure in both groups had valvelike openings of the atrial septum that closed by fusion of the valves. ASDs that closed spontaneously in infants with clinical signs and symptoms of atrial shunt may be the same clinical entity as the valvelike openings of the atrial septum frequently observed in neonates with few clinical symptoms and may be differentiated morphologically from ASDs with openings without valve formation, which have little tendency to close.