Aims: Our aims were to use transthoracic three-dimensional echocardiography to assess the morphology of atrial septal defects in children prior to closure, and to compare the three-dimensional echocardiographic data with transcatheter and surgical findings. Methods and results:We used transthoracic three-dimensional echocardiography in 62 consecutive patients, aged from 2 to 18 years, with atrial septal defects, measuring the maximal diameter and the extent of the rims. Subsequent to the study, we referred 42 patients for transcatheter closure, the rims being measured at greater than 4 mm. We found a good correlation between the maximal diameter of the defect as measured at transthoracic three-dimensional echocardiography and using a balloon (y = 3.45 − 0.73x; r = 0.78; p < 0.0001), the mean difference between the measurements being 2.4 ± 2.8 mm. Successful closure with the Amplatzer septal occluder, having a mean size of 22 ± 4 mm, was achieved in 95% of the patients. Of the original cohort, 20 patients were referred for surgical closure. In these patients, the inferior rim had been deemed insufficient in 5, the postero-superior rim in 6, and the postero-inferior rim in 9. Complete agreement was found when the deficiency of the rim as judged using transthoracic three-dimensional echocardiography was compared with intraoperative findings. The correlation between measurements of the deficiency of the rim achieved by transthoracic three-dimensional echocardiography and at surgery was excellent (y = 0.2 + 0.98x; r = 0.93; p < 0.0001), the mean difference between the measurements being no more than 0.6 ± 0.4 mm. Conclusions: Transthoracic three-dimensional echocardiography proved accurate in measuring the maximal diameter and rims of atrial septal defects within the oval fossa. This non-invasive method will be valuable in selecting children for transcatheter or surgical closure of such defects.