A 52-year-old man was admitted to the hospital because of unstable angina pectoris. Coronary angiography revealed severe stenosis at a proximal site of the left anterior descending artery. Essential thrombocythemia (ET) was diagnosed on the basis of findings of marked thrombocytosis (106 x 10(4)/microL) and an increased number of immature megakaryocytes in the bone marrow. Because hyperaggregability of platelets was demonstrated by an ex vivo platelet aggregation assay and by elevated plasma levels of beta-thromboglobulin (beta-TG) and platelet factor 4 (PF4), antiplatelet therapy with aspirin and ticlopidine and cytoreduction therapy with hydroxyurea were started. This combination treatment resulted in a decrease in the platelet count to less than 60 x 10(4)/microL and decreases in plasma levels of both beta-TG and PF4 to almost normal values. Percutaneous coronary angioplasty and stenting were then performed successfully without thrombotic complications. These findings suggest that combination therapy with antiplatelet and cytoreduction agents before catheter intervention is useful for the prevention of thrombotic complications in patients with acute coronary syndrome associated with essential thrombocythemia.