Abstract Background Elderly patients represent a large proportion of patients admitted for Acute Coronary Syndrome (ACS). Whether frailty, defined as a biological syndrome that reflects a state of decreased physiological reserve and vulnerability to stressors, may impact the clinical outcomes in this population remains unclear. We aimed to determine the prevalence of frailty and its impact on mortality in patients aged ≥ 80 years admitted for ACS. Methods This prospective observational study was conducted among patients aged 80 or older admitted in a tertiary hospital for ACS. Frailty was assessed using the Edmonton Frail Scale (EFS) which provides a score ranging from 0 (not frail) to 17 (very frail). Population was divided into 3 classes: 0-3 EFS-score, 4-6 EFS-score, and >7 EFS-score. Results 236 patients were included with a mean follow-up duration of 470 days. The mean age was 85.9 years. 75 patients died during follow-up period.119 subjects (50.4%) had a 0-3 EFS-score, 68 patients (28.8%) had a 4-6 EFS-score and 49 patients (20.8%) had a ≥ 7 EFS-score. All-cause mortality rate was 17.7% in the 0-3 EFS-score group, 35.3% in the 4-6 EFS-score group and 61.2% in the ≥ 7 EFS-score group, (p<0.001). After multivariate analysis, frailty status remained associated with all-cause mortality: HR was 1.53 (95% CI 0.74 - 3.16) within the 4-6 EFS-score group, and HR was 3.60 (95% CI 1.70 - 7.63) within the ≥ 7 EFS-score group. Conclusion Frailty is a strong and independent prognosis factor of midterm all-cause mortality in elderly patients presenting with ACS.