Coronary angioscopy has visualized and elucidated the pathophysiology of various coronary artery diseases, especially, acute coronary syndrome (ACS). Culprit lesions of ACS can be diagnosed as the irregular-surfaced yellow plaques with a predominantly white thrombus. Yellow material covered by white thrombus is often observed protruding from the plaque, fragmented and washed away into the distal artery. This lipid-rich material mixed with white thrombus has been shown to cause distal embolization and no-reflow phenomenon, and has recently become the therapeutic target of distal protection devices. Angioscopy can easily detect vulnerable plaques as yellow plaques. We found that yellow plaques of higher yellow color intensity tended to have thrombosis more frequently. However, it may be difficult to find out a dangerous plaque from the whole coronary artery trees. The number of yellow plaques in a major coronary artery or the yellow color intensity of those plaques may be a marker of coronary atherosclerosis and may be a risk factor of ACS. We thought it would be easier to find out patients than plaques at high risk of ACS. We followed up 85 patients of acute myocardial infarction for about 5 years and found that patients who suffered another ACS event had more yellow plaques of higher yellow color intensity at baseline than those who did not. Direct visualization by angioscopy provides rather subjective information but may give some indication of the pathophysiology or the need for and success of treatment of coronary disease.