Atrial septal defects (ASDs) are found more frequently in the pediatric population than in adults, and improved diagnostic techniques with echocardiography (2DE) and Doppler facilitate diagnosis so that repair is possible at an optimal time. The purpose of our investigation was to study the size of ASDs at diagnosis, how size changes during follow-up, and to explore the relationship between size at diagnosis and need for surgery. We reviewed the medical records of all patients in Iceland with the diagnosis of ASD born between 1984 and 1993. ASD was confirmed by 2DE in all patients and defects smaller than 4 mm were excluded. ASD size was measured by 2DE from subxyphoid long and short axis views. There were 91 patients-29 males and 62 females. Four patients died from causes other than the heart defect and had not been operated upon. Seven patients with ASD primum and sinus venosus defects were excluded from analysis. There were 29 patients with a 4 mm defect, 17 patients with 5 or 6 mm defects, 8 patients with 7 or 8 mm defects, and 26 patients had defects >8 mm. In the 4 mm group, in 26 patients (89%) the ASD closed spontaneously or decreased in size, and 1 patient had been operated upon. In the 5 or 6 mm group, 15 of 19 ASDs (79%) had closed spontaneously, and 2 patients (9.5%) had been operated upon. In the 7 or 8 mm group, 1 of 6 ASDs (16.6%) had closed spontaneously and 3 had been closed surgically. In the >8 mm group, 1 of 24 ASDs had closed spontaneously and 20 (91%) had been closed surgically. We conclude that defects smaller than 6 mm in diameter are very likely to close spontaneously although follow-up is necessary. Defects larger than 8 mm have a high probability requiring operative closure.