Thrombohemorrhagic disorders are the main cause of morbidities and mortalities of essential thrombocythemia (ET), which are typically observed at age 50–60 years and rarely encountered in adolescence or childhood. Recently, anagrelide, a quinazinolone derivative, has been used as a therapeutic agent for ET. Although it is used to reduce platelet count, its cardiotoxicity has been reported. Here, we present an 18-year-old boy with ET who was treated with anagrelide and developed acute myocardial infarction. This was presumed to be an effect of anagrelide administration and, specifically, damage to the coronary arterial endothelial cell exacerbated by ET.