Acute myocardial infarction (AMI) is relatively rare in systemic lupus erythematosus (SLE), although other cardiac complications, such as pericarditis and myocarditis, occur frequently in this disease. A 20-year-old woman with documented SLE experienced a transmural anterior AMI due to thrombi in saccular aneurysms of the left main coronary artery and the proximal portion of the left anterior descending coronary artery. There were also saccular and fusiform aneurysms in the right coronary artery, but thrombi were not observed in them. Aorto-coronary bypass surgery was performed to salvage the viable myocardium and to prevent recurrent myocardial infarction and rupture or infection of these coronary aneurysms. Postoperative coronary angiography revealed a new small saccular aneurysm in the mid-portion of the right coronary artery. During this period, there was no immunological evidence of active SLE. It is important to ascertain whether such coronary aneurysms resulted from atherosclerosis or arteritis, because of the choice of the different therapeutic interventions. In this case, however, it was difficult to determine. It was speculated that these coronary aneurysms arose from an arteritic process, because the saccular aneurysm in the mid-portion of the right coronary artery was formed in less than three months, there were no coronary risk factors, and any microscopic evidence of atherosclerosis was not obtained in the aortic specimen during aortocoronary bypass surgery. Serial coronary angiographic studies are necessary for accurately diagnosing coronary artery disease. Anticoagulant therapy and antiinflammatory medication may be necessary to prevent myocardial infarction in patients with SLE, even if there is no immunological evidence of active SLE.