A 45 years old man presented to the emergency room with severe squeezing chest pain. He had mechanical prosthetic valve for 5 years and was diagnosed as cerebellar infarction before 3 years. A few days ago, he stopped taking warfarin for extraction of tooth. A 12 lead electrocardiogram(ECG) showed ST segment chages and Q waves of an acute anterior myocardial infarction and arterial fibrillation. Coronary angiography was done. There was diffuse thrombi in the left main coronary artery that was not recanalised by direct percutaneous transluminal coronary angioplasty(PTCA). So thrombolytic and anticoagulant therapy was performed after PTCA. Coronary angiography was carried out 3 and 10 days after thrombolysis and heparinization. A thrombi in left main coronary artery disappeared. His serum anti-lupus anticoagulant antibody was positive and anti-cardiolipin antibody IgG was high(>120 GPLU/mL). Primary antiphospholipid syndrome should be considered as a cause of acute myocardial infarction, and PTCA with thrombolytic treatment is effective for initial treatment of the syndrome. After discharge, he has since remained well and asymptomatic with taking warfarin. Keywards : Primary antiphospholipid syndrom, acute myocardial infarction.