The occurrence of thrombotic events is central to the course of polycythemia vera. Myocardial, cerebral, peripheral, and pulmonary infarctions are frequent and are consequences of throbmoses in small and medium caliber arteries. Thrombosis in large caliber arteries is a rare event. Thrombosis within the chambers of the heart has not been hitherto reported. This report documents the occurrence of massive left ventricular thrombosis in a patient with polycythemia vera. The throbmus reduced the left ventricular capacity by about 75% and caused intractable congestive heart failure.