Neoplastic involvement of the pericardium occurs frequently in post-mortem studies. Symptoms of this involvement of the pericardium prior to death are unusual. This report describes a case of breast carcinoma in a 54-year-old woman who presented with cardiac tamponade. Because breast carcinoma as a cause of cardiac tamponade is unusual, its presentation is reported. Early recognition of the possibility of malignancy allows for initiation of appropriate treatment, thereby improving the patient's chance of survival. It is recommended that all patients who present with tamponade have pericardial fluid sent for cytologic examination.