Objectives: We evaluated the influence of complex conditions on the results of transcatheter atrial septal defect (ASD) closure using Amplatzer Septal Occluder (ASO) in most recent years. Methods: From June 2003 to April 2005, 148 patients underwent transcatheter closure of ASD with ASO. Complex conditions, such as the presence of a large defect (> 26mm, n=40), multiple defects (n=7), rim deficiency (< 5mm, n=82), need of additional procedures (balloon valvuloplasty, n=19) and very young age (< 3 years, n=32), were defined and the influence of these conditions on results of ASD closure was analyzed by comparison of success rate, residual shunt rate, procedure time, fluoroscopy time and rate of complication. Results: Implantation of the ASO was successful in 147(99%) patients. Echocardiographic evaluation at 24 hours after the procedure revealed 9 patients(6.2%) had residual shunts, but only 2 patients(1.4%) showed small residual shunts on the follow-up evaluation from 1 to 20 months after the procedure. Mean procedure and fluoroscopy time (minutes) was 82.9짹17.6 and 31짹10.9. No major complication occurred during the procedure. There were 3 cases of transient atrial tachycardia and 3 cases of groin hematomas after procedures. Multivariate and univariate analyses revealed that 1) no factor had influence on the procedural success rate (P>0.05). 2) patients with deficient rim had residual shunt more frequently only at the 24-hour follow-up (P=0.028). 3) procedure and fluoroscopy time were longer in patients with large or multiple defects, additional procedures and very young age (P=0.026~0.037). Conclusion: Transcatheter closure of secundum ASD with ASO is technically feasible, safe and effective even in the presence of complex conditions. These conditions may not influence the final success rate and morbidity.