PURPOSE: To know the natural course of the ostium secundum ASD including its rate of spontaneous closure. METHODS: We studied 259 cases who were diagnosed as ostium secundum ASD by 2D-echocardiography at Department of Pediatrics, Kyungpook National University Hospital between Jan. 1985 and Dec. 1994. According to the age at diagnosis, we divided them into two groups, group I (those diagnosed before 1 month of age: 59 cases) and group II (those diagnosed after 1 month of age: 200 cases) RESULTS: Mean age at first diagnosis was 2.6+/-4.6 years and mean defect size(diameter by 2D-echocardiography) was 9.9+/-8.0 mm. Cardiac murmur was the main reason to refer the patients to hospital, being in 50 cases(87%) in group I, 179 cases(90%) in group II. Defect sizes were significantly small in group I compared with group II (5.3+/-3.4 mm vs 11.1+/-6.9 mm, p<0.05). Symptoms corresponding to congestive heart failure were present in 16 cases of group I (27%)( isolated ASD: 8 cases, ASD accompanying with PDA: 5 cases, ASD accompanying with VSD: 2 cases, ASD accompanying both PDA and VSD : 1 case) and in 27 cases of group II(13.5%) (isolated ASD: 17 cases, ASD accompanying with VSD: 4 cases, ASD accompanying with PDA: 4 cases, ASD accompanying both PDA and VSD: 2 cases). Of total 139 cases, spontaneous closure of ostium secundum ASD was documented in 14 cases(10.1%) by follow up 2D-echocardiography. The rate of spontaneous closure was significantly high in group I compared with group II (6 cases: 37.5% vs 8 cases: 6.5%, p<0.05) Comparing with those cases without closure, there were no significant differences in defect size in group I but significant differences in group II(p<0.05), being 5.63 2.62 mm in cases with spontaneous closure and 12.82 7.09 mm in those cases without closure. Of the 9 cases with atrial septal septal aneurysm, 2D- echocardiographic follow ups were possible in 5 cases with spontaneous closure in 2 (40%). CONCLUSIONS: Spontaneous closure was documented in 10.1% of all the patients with ostium secundum ASD. Especially the closure rate was significantly high in those cases their diagnosis was made before 1 month of age and with atrial septal aneurysm.