Coronary artery aneurysms are uncommon, usually associated with atherosclerosis and rarely involve all 3 main coronary arteries. Sudden death from documented thrombosis within large coronary aneurysms has been rarely reported. The authors report a case of a previously healthy 36-year-old male who presented with myocardial infarction complicated by sudden cardiac death. The patient was successfully resuscitated, and coronary angiography revealed diffuse, severe aneurysmal disease without evidence of atherosclerosis. A thrombus was visualized in a large aneurysm of the proximal left anterior descending artery, and there was total occlusion of a second diagonal branch, presumably due to thrombus embolization. The patient had no history of Kawasaki disease, and evaluation revealed no inflammatory or autoimmune condition. Optimal treatment and prognosis for patients with nonatherosclerotic coronary aneurysms remains unclear. Our patient was treated medically with chronic warfarin and low-dose aspirin therapy and recovered without complication.