The two factors responsible for the development of left ventricular mural thrombi are endocardial injury secondary to old or recent anterior myocardial infarction and left ventricular dysfunction. Endothelial damage also is thought to be the initial event in the development of arterial thrombi. However, arterial thrombi may develop in patients with thrombocytosis secondary to myeloproliferative disorders in the absence of endothelial injury. A patient had thrombocytosis secondary to agnogenic myeloid metaplasia and a left ventricular mural thrombus developed in the absence of clinical or laboratory evidence of old or coronary angiogram and left ventricular function. To our knowledge, this is the first such case reported.