Aneurysms of the coronary arteries occur in 1.5-4.9% of coronary angiograms. Only a few cases of a giant aneurysm of the left main coronary artery in conjunction with severe atherosclerotic coronary disease have been reported. This report details the exceptional case of a 72-year-old patient with a giant "true saccular" aneurysm branched on the left main coronary artery. The development of an aneurysmal dilatation in conjunction with progression of coronary arteriosclerosis, observed from two different angiographic studies at an interval of 25 years, is very illustrative for evolutive atherosclerotic heart disease. This finding may support the hypothesis that atherosclerosis may be the common etiology for coronary aneurysmal dilatation.