Background. Transcatheter closure of secundum atrial septal defect (ASD) is accepted as a safe alternative method to surgery. However, the deployment of the device across the septum using the conventional method presents difficulties in large ASDs. Methods. In the modified balloon-assisted technique (modified BAT), a Tyshak balloon (Numed, Inc) is placed in one of the left pulmonary veins and a long sheath is placed in the right upper pulmonary vein. Through this sheath, the Amplatzer septal occluder is deployed to its proper position with the help of the Tyshak balloon, which prevents the left atrial disc from prolapsing into the right atrium. This method was implemented in the treatment of patients with a large secundum ASD. In this study, defects that measured >= 15 mm by transesophageal echocardiography (TEE) were considered to be large secundum ASDs. Between June 2011 and September 2013, the modified BAT was used in the closure procedure of 30 patients (18 females/12 males) with large ASDs (group I). Between September 2003 and September 2013, 78 patients (47 females/31 males) with large ASDs were treated with conventional methods (group II). The results of both groups were compared. Results. In the comparison of the results between group I and group II, there were no significant differences between mean maximal ASD sizes by transthoracic echocardiography, mean maximal ASD size by TEE, Qp/Qs ratio, mean device diameter, failure rate, embolization incidences, residual shunt, and fluoroscopy time. However, there were significant differences in the mean ages, mean body weights, and mean PAPs between group I and group II. After adjusting for age and body weight, in terms of process failure, the conventional method was found 5.6 times more risky compared to modified BAT. Conclusion. Large secundum ASDs in children, transcatheter closure with Amplatzer septal occluder using the modified BAT is a simple, quite useful method. In addition, this method seems to reduce the procedure failure rate.