A five-year-old Holstein cow presented to the Cornell University Equine and Farm Animal Hospital for a five day history of depression, poor appetite, tachypnea, and reduced milk production. Further evaluation revealed venous distention and muffled heart and lung sounds. The cow was negative on a BLV ELISA and no evidence of wire foreign body was found on radiography. An electrocardiogram revealed a normal sinus rhythm with electrical alternans. An echocardiogram was performed which revealed a structurally normal heart and pericardial effusion resulting in cardiac tamponade leading to right-sided heart failure. Pericardiocentesis was performed and resolution of clinical symptoms resulted. Cytologic evaluation of the effusion was consistent with idiopathic pericardial effusion.