Idiopathic hypereosinophilic syndrome (HES) is a rare multisystem condition characterized by dysregulated overproduction of eosinophils. Cardiac involvement in HES is characterized by necrosis from infiltration of eosinophils and thrombus formation and, in the late stage, by fibrosis and chronic valvular regurgitation. We report a very unusual presentation of idiopathic HES with acute mitral regurgitation due to papillary muscle rupture. The transesophageal echocardiogram was suggestive of a flail posterior leaflet and suspicious for endocarditis. Intraoperatively, papillary muscle rupture was seen and the patient underwent mitral valve replacement. The pathologic examination of the valve revealed eosinophilic infiltration of the papillary muscle. The patient was treated with steroids and responded well clinically.