BACKGROUND: Women experience poorer health outcomes following acute coronary syndrome (ACS). Heart rate (HR) and heart rate variability (HRV) have emerged as sensitive and cost-effective markers of autonomic function and prognostic risk factors of poor cardiac outcomes. The aim of the current study was to investigate whether sex-specific differences existed across HR and five parameters of HRV, at 1 and 12 months following ACS diagnosis. METHODS: Between January 2013 and June 2014, a sample of 416 ACS patients was enrolled in the Anxiety Depression & Heart Rate Variability in cardiac patients: Evaluating the impact of Negative emotions on functioning after Twenty four months (ADVENT) longitudinal cohort study. At 1 and 12 months following discharge, patient HR and HRV (root mean square of successive differences [RMSDD], standard deviation of RR intervals [SDRR], high frequency power [HF], low frequency power [LF], very low frequency power [VLF]) was measured via three-lead electrocardiogram. RESULTS: At 1 month post-ACS, sex was a significant predictor of HR and VLF power in fully- adjusted models. At 12 months post-ACS, sex was a predictor of HR, SDRR and VLF power in fully-adjusted models. CONCLUSION: Sex-specific differences in resting HR and HRV were observed in the year following ACS, whereby women had higher HR and lower HRV, suggestive of poorer autonomic function. Further large-scale cohort studies examining autonomic function as a driver of sex-specific outcomes following ACS are required.