Aneurysms of the coronary arteries occur in from 0.3% to 4.9% of angiograms. Only 12 cases of left main artery (LMA) aneurysms have been reported. Of these, seven were associated with atherosclerosis in patients more than 56 years old. This report details the case of a 39-year-old patient with a large LMA aneurysm associated with atherosclerosis.