Unrepaired secundum ASD (atrial septal defect) is the commonest congenital heart disease encountered in adulthood. In this study we try to correlate the three-dimensional size of a secundum ASD to the device size. We did a retrospective analysis to 36 patients who underwent successful transcatheter closure of atrial septal defect guided by balloon sizing and correlated this to the device size. The device size was correlated significantly with all the maximal ASD diameter by 2D-TTE, 2D-TEE and 3D-TEE. However, the most significant correlation was between three-dimensional maximal diameter and device. 3D TEE is a promising and reliable tool that can be used to determine the size, number and shape of ASD and to accurately select the suitable device size during percutaneous closure.