Introduction: Delayed pericardial effusion following WATCHMAN FLX implantation is not well understood. We present a case series of two patients with hyperdynamic left atrial appendages (LAA) who developed delayed pericardial effusions following WATCHMAN FLX™ implantation. Methods: Successful left atrial appendage closure was performed in 2 patients with paroxysmal atrial fibrillation using the WATCHMAN FLX™ device. Both patients had hyperdynamic left atrial appendages noted during their procedures. Intra-procedure course was uncomplicated. Results: Delayed pericardial effusion occurred necessitating drainage in both cases. Conclusion: While exact mechanisms remain unclear, sinus rhythm and the presence of a hyperdynamic LAA may play a role. Understanding the interaction of the appendage and the delivery system may help to reduce the likelihood of this rare, but serious, complication. Keywords: Atrial fibrillation; Left Atrial Appendage Closure; Pericardial Effusion; Pericardiocentesis.