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Background: The involvement of cholesterol crystals (CCs) in plaque progression and destabilization of atherosclerotic plaques has been recently recognized. This study aimed to evaluate the association between the intraplaque localization of CCs and plaque vulnerability. Methods: We investigated 55 acute coronary syndrome (ACS) and 80 stable angina pectoris (stable AP) lesions using optical frequency domain imaging (OFDI) prior to percutaneous coronary intervention. The distance between CCs and the luminal surface of coronary plaques was defined as CC depth. Results: Although the incidence of CCs had similar frequencies in the ACS and stable AP groups (95% vs. 89%, p = 0.25), CC depth was significantly less in patients with ACS than in those with stable AP (median [25th to 75th percentile]: 68 μm [58 to 92 μm] vs. 152 μm [115 to 218 μm]; p < 0.001). The incidences of plaque rupture, thrombus, lipid-rich plaques, and thin-cap fibroatheroma were significantly greater in patients with ACS than in those with stable AP (62% vs. 18%, p < 0.001; 67% vs. 16%, p < 0.001; 84% vs. 57%, p < 0.01; and 56% vs. 19%, p < 0.001, respectively). Conclusion: OFDI analysis revealed that CCs were found in the more superficial layers within the coronary atherosclerotic plaques in patients with ACS than in those with stable AP, suggesting that CC depth is associated with plaque vulnerability. CC depth, a novel OFDI-derived parameter, could be potentially used as an alternative means of evaluating plaque vulnerability in coronary arteries. [ABSTRACT FROM AUTHOR]