Abstract Background/Introduction There is an alarming rise in the incidence of atherosclerotic coronary artery disease (CAD) among young adults, particularly in young women. Sex disparities of early-onset CAD in the context of contemporary cardiovascular practice are not well understood. Purpose We aimed to delineate the differences in clinical profiles and prognosis between young men and women with established early-onset CAD. Methods This is a nationwide, hospital-based, longitudinal prospective cohort study. Young patients (aged ≥18 years and ≤ 45 years) with angiographically confirmed severe atherosclerotic obstructive coronary arteries (luminal narrowing ≥ 70%) were included from 38 hospitals across China between May 2017 and May 2020. Coronary risk factors, clinical features and treatments were compared between men and women at baseline. Participants were followed for 3 years. The outcome was major adverse cardiovascular events (MACE), a composite of cardiovascular death, non-fatal myocardial infarction (MI), unplanned revascularization, and ischemic stroke. The Cox regression analyses were used to estimate hazard ratios (HR) with 95% confidence intervals (CIs) for outcome. Results There were 1524 patients (female: 4.4%) included in this study. The female patients were older than the male patients (41.1±4.0 years for females vs. 39.6±4.4 years for males, p=0.006). They showed a lower prevalence of hypertension, lipid disorders, and smoking or drinking (all p<0.01). The female patients were less likely to have at least 1 modifiable risk factors (female: n=45 (67.2%) vs. male: n=1382 [94.6 %], p<0.001); they showed more favorable laboratory results at baseline, including lipids, serum creatinine and uric acid (all p<0.05) as compared to male patients. Besides, young females presented more often with stable angina on admission, whereas young males tended to present with acute MI (p<0.001). There was no significant sex difference in Gensini Score (p=0.15). Female patients were less likely to undergo revascularization procedures than male patients (73.1% vs. 85.3%, p=0.01). During the 3 years of follow-up, female patients were at a significantly higher risk of MACE compared to their male counterparts (13.4% vs. 5.8%, adjusted HR: 3.07, 95% CI:1.36–6.94, p=0.007) after adjusting for age, smoking, drinking, body mass index, hypertension, diabetes, dyslipidemia, diagnosis and revascularization. Conclusions Distinct sex disparities, unfavorable to young females, were documented among patients with early-onset CAD, with the largest gaps noted in the treatment and prognosis. Despite more favorable baseline profiles, young female patients had insufficient care and 3-fold greater risk for MACE compared to young males. These results highlight the need for strengthening the treatment strategies for this vulnerable disease entity.Figure 1