A 20 year old man with severe chest pain was hospitalised for acute myocardial infarction. Coronary angiography revealed total obstruction of his right coronary artery, which was successfully recanalised by direct percutaneous transluminal coronary angioplasty (PTCA). There was also diffuse thrombi in the left coronary artery that was not recanalised by perfusion with 3000 U pro-urokinase. Anticoagulant therapy was performed after PTCA. Creatine kinase peaked one day after hospitalisation (4805 U/l). The activated partial thromboplastin time was 62.6 seconds (45%). Plasma anticardiolipin IgG antibodies were high (3.8 and 2.7) in repeated examinations. The PTCA site was patent after three months. Primary antiphospholipid syndrome should be considered as a cause of acute myocardial infarction in young adults, and PTCA with anticoagulant treatment is effective for initial treatment of the syndrome.