BACKGROUND Acute coronary syndrome(ACS) is linked to a range of in-hospital complications, and age is recognized as risk factor for adverse events. Discrepancies between physiological and chronological age are explained by frailty. However, the relationship between frailty and in-hospital complications is not clear.METHODS Assessment of frailty in patients was carried out using the FRAIL scale. In-hospital complications assessed included, bleeding, infection, arrhythmia, acute kidney injury(AKI), delirium, stroke/transient ischemic attack(TIA), liver injury,hypoglycemia, length of stay in the cardiac care unit(CCU).RESULTS Of the 174 patients, frailty was identified in 39.1% and pre-frailty in 29.9%. Frailty was associated with a higher incidence of all types of bleeding(frail vs. robust: 45.5% vs. 16.7%, P < 0.001) and infection(54.4% vs. 11.1%, P < 0.001), including pneumonia/lower respiratory tract infections(LRTI) and urinary tract infections(UTI). Incidence of antibiotic therapy(52.9% vs.13.0%, P < 0.001), atrial fibrillation(AF)(47.1% vs. 9.3%, P < 0.001), AKI(57.3% vs. 20.4%, P < 0.001), delirium(52.9% vs. 3.7%, P <0.001), liver injury, were higher in frail patients(17.6% vs. 0, P = 0.001), whilst their length of stay in the CCU was longer(4 days(2-6.5) vs. 2 days(2-3), P < 0.001). Infections, pneumonia/LRTI, antibiotic therapy during hospitalization, the incidence of AF and liver injury were more often in patients with pre-frailty compared to the robust group. After adjustment for potential confounders, frailty remained independently associated with an increased risk of infection(OR: 3.3 [1.6-7.0]), including pneumonia/LRTI(OR: 2.5 [1.1-5.8]) and UTI(OR: 4.8 [1.8-12.5]). Frail individuals had an increased requirement for antibiotic therapy(OR: 3.9[1.9-8.1]), and greater risk of AF(OR: 3.5 [1.3-9.3]), AKI(OR: 2.6 [1.2-5.3]) delirium(OR: 11.7 [4.8-28.7]), as well as having to stay longer in the CCU(> 3 days)(OR: 3.7 [1.9-7.3]).CONCLUSIONS Frailty was associated with an increased risk of numerous in-hospital complications in elderly patients who had been hospitalized with ACS.