A 67 year-old man presented with stable angina pectoris. He underwent primary angioplasty with stenting of the left anterior descending artery ostial lesion five months previously. A small eccentric aneurysm was shown within the left main coronary artery. A second angiography, performed a month later, showed slight enlargement of the aneurysm from the base side. In the final presentation, angiography revealed spontaneous dissection line beginning from the aneurysm base and extending to the left anterior descending artery. Although progression from coronary aneurysms to dissection is rare, it can occur, and patients monitored closely, especially when left main coronary aneurysm is present.