The Amplatzer septal occluder (ASO) is used for transcatheter closure of atrial septal defects (ASDs). This study aimed to determine the factors influencing successful closure with the ASO. A retrospective analysis of 69 patients who underwent transcatheter ASD occlusion between 2003 and 2007 was performed. The ASO was successfully implanted during 67 (97%) of 69 procedures. A major adverse event occurred for 6 patients (9%), and 13 patients (19%) experienced a minor adverse event. The outcome for 53 cases (77%) was a composite clinical success. Patient age (p = 0.191) and consultant experience (p = 0.270) were not important factors in successful ASD occlusion. However, patient weight (p = 0.031), diameter of the defect (p = 0.030), device size (p = 0.044), aortic rim size (p = 0.002), and device/defect ratio (p < 0.001) all were significant factors. Complications were significantly more likely for patients whose device/defect ratio was <1.125 (loose) or ≥1.333 (tight) (p < 0.001). The device/defect ratio may provide a clinically useful tool with the potential to predict patients likely to experience an adverse event as a result of transcatheter ASD occlusion. A larger sample would enable refinement of the device/defect ratio and provide a more robust prediction of success.