The other writing committee members and coauthors declare that there is no conflict of interest.

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Background: Despite the significant decline in cardiovascular mortality in women over the past several decades, sex differences in the underlying pathology of acute coronary syndromes remain poorly understood. Previous postmortem studies have demonstrated sex differences in coronary plaque morphology with a higher prevalence of plaque erosion in young women and more plaque rupture in older women after menopause, whereas men showed no increase in prevalence of plaque rupture with age. However, in vivo data are limited. Methods: This study included patients who presented with acute coronary syndrome and underwent preintervention optical coherence tomography imaging of the culprit lesion. The culprit plaque was categorized as plaque rupture, plaque erosion or culprit plaque with calcification, and stratified by age. Features of plaque vulnerability at culprit lesion were also analyzed. Results: In 1368 patients (women=286), women and men had a similar distribution of culprit plaque morphology (plaque rupture versus plaque erosion). However, significant sex differences were found in the underlying mechanisms of acute coronary syndrome among different age groups: women showed a significant ascending trend with age in plaque rupture ( P <0.001) and the features of plaque vulnerability such as lipid plaque ( P <0.001), thin-cap fibroatheroma ( P =0.005), and microstructures including macrophages, cholesterol crystals, and calcification ( P =0.026). No trend was observed in men. Conclusions: Age related sex differences in culprit plaque morphology and vulnerability were identified in patients with acute coronary syndrome: prevalence of plaque rupture and vulnerability increased with age in women but not in men. Registration: URL: https://www.clinicaltrials.gov ; Unique identifier: NCT01110538 and NCT03479723.