Abstract Body (Do not enter title and authors here): Cardiac rehabilitation (CR) is recommended for ST-elevation myocardial infarction (STEMI) patients, yet is commonly underutilized. Past research suggests that disparities exist in CR referrals and participation by patient characteristics. The aim of this study was to assess sex differences in CR referrals, participation and completion in STEMI patients. Methods: A retrospective cross-sectional analysis of 1098 STEMI patients (306 females and 792 males) between January 1, 2022 and December 31, 2024 was performed. Sex differences in CR referral, participation, completion (attending ≥ 12 sessions), and exercise progression (change in aerobic capacity measured by maximal metabolic equivalents (METs) at baseline and session 12) were assessed. Logistic regression models were run to assess predictors of CR referral, participation, and completion adjusted for patient (sex, age, race/ethnicity, language, rural/urban residence, insurance) and clinical factors (comorbidities). Results: During the study timeframe, 74.0% of STEMI patients received CR referrals. Female STEMI patients were 10.1% less likely to receive a CR referral compared to male patients. Of those referred, female patients were also less likely to participate (23.1% vs. 31.6%) and complete CR compared to males (Of those enrolled, 44.0% vs. 58.1%, respectively). Finally, females that completed CR experienced less improvement in their aerobic capacity compared to males (delta Maximal METs: 0.56 vs. 1.27, respectively). Results from the unadjusted logistic regression models found sex to be a significant predictor of CR referral (OR: 1.59 [95% CI: 1.10 – 2.29]) and participation (OR: 1.59 [95%CI: 1.09 – 2.34]). Sex remained a significant predictor of CR referral in the adjusted model (OR: 1.44 [95% CI: 1.02 – 2.04]) but was no longer significantly associated with CR participation (Table 1). Sex was not significantly associated with CR completion. Conclusion: Female STEMI patients experienced lower CR referral and participation rates, as well as reduced improvements in aerobic capacity compared to males. Given the well-established benefits of CR on patient outcomes, quality improvement initiatives are needed to understand these disparities and increase the percentage of all eligible patients participating in and completing CR.