A 56-year-old female with idiopathic hypereosinophilic syndrome who presented with chest tightness, dyspnea and new-onset atrial fibrillation is reported. Work up revealed significant peripheral eosinophilia, pericarditis, and a pericardial effusion with tamponade physiology. Pericardiocentesis revealed predominant eosinophils (88%) in the pericardial fluid. After initial stabilization, pericardial effusion reaccumulated with persistence of marked eosinophilia. A diagnosis of idiopathic hypereosinophilic syndrome was made, and she was started on prednisone with a dramatic resolution of her clinical symptoms.