Coronary artery aneurysm is a rare coronary abnormality, usually diagnosed incidentally by coronary angiography. Major causes of coronary aneurysms include coronary ectasia, Kawasaki disease, and atherosclerosis. Most of the discrete coronary aneurysms are of atherosclerotic origin. The incidence of atherosclerotic coronary aneurysms is about 0.2%, and the left main coronary artery is the least frequently involved artery. Only a few cases of left main coronary artery aneurysm have been reported in the literature, and a left main coronary artery aneurysm involving the proximal segments of the left anterior descending and the left circumflex arteries has not been reported previously. The authors describe this finding in a man who presented with worsening exertional angina pectoris. Coronary angiography demonstrated an aneurysm of the distal left main coronary artery extending into the proximal segments of the left anterior descending and the left circumflex arteries. In addition, a significant flow-limiting atherosclerotic lesion was present in the proximal portion of the left anterior descending artery distal to the aneurysm.