Introduction: Pericardial effusion (PE) without obvious periprocedural complications (e.g., cardiac perforation, device erosion) may occur after transcatheter closure of secundum atrial septal defects (ASD).It has only been described in case reports, and has not been the subject of systematic study. Hypothesis: The aim of the study is to investigate the incidence and predictors of PE unrelated to procedural complications, thus providing interventional cardiologists with a novel way to approach new-onset PE after ASD closure. Methods: We included all patients who had undergone successful percutaneous ASD closure from June 2009 to April 2014 (n=2652) with no preexisting PE or cardiac perforation or erosion. Transthoracic echocardiography (TTE) was performed during the procedure and 1, 3, and 6 months postoperatively. Results: After device implantation, fifty patients (1.9 %) developed new-onset PE (37 immediately, 13 during follow-up). These patients were asymptomatic, stable hemodynamically, and had no new arrhythmias. PE appeared mild (5.1±1.9 mm) and homogeneously echo-lucent by TTE (Figure). PE diminished spontaneously. Compared with 2602 patients without PE, factors independently predicting asymptomatic PE were the device touching the atrial free wall, device size, patient age, and total defect size (Table). Areas under the receiver operating characteristic curves were 0.78 (p<0.001), 0.66 (p<0.001) and 0.77 (p<0.001) for device size, patient age, and total defect size, respectively. Conclusion: A new type of PE is firstly reported systematically. Our data provide new insights into new-onset PE after percutaneous ASD closure.