Abstract Background Although the incidence of acute myocardial infarction has been declining over the past decade, the etiology of this trend is unknown. One possible explanation could be changes in coronary plaque phenotype. Purpose The aim of this study was to investigate the changes in plaque characteristics over the last 10 years. Methods Patients who presented with acute coronary syndrome and underwent preintervention optical coherence tomography (OCT) imaging from 2008 to 2018 were included. Patients were divided into 4 periods: (P1) 2008-2010, (P2) 2011-2013, (P3) 2014-2015, and (P4) 2016-2018. Plaque characteristics at culprit lesion in each period were compared. Results Among 1144 patients, 594 (51.9%) presented with ST-segment–elevation myocardial infarction (STEMI). The prevalence of thin-cap fibroatheroma decreased from P1 to P4 in both STEMI group (57.4% to 24.6%, P<0.001) and non-ST-segment elevation acute coronary syndrome (NSTE-ACS) group (40.4% to 17.0%, P<0.001) (Figure). Conclusions The prevalence of OCT features of plaque vulnerability has decreased over the past 10 years. A decrease in plaque vulnerability may explain the favorable trend in cardiovascular events.Prevalence of thin-cap fibroatheroma