There is a paucity of evidence supporting routine beta blocker (BB) use in patients undergoing percutaneous coronary intervention (PCI) for acute coronary syndrome (ACS). The aim of this study was to evaluate BB use post PCI and its association with mortality. Furthermore, the study aimed to evaluate the association between BB and mortality in the subgroups of patients with left ventricular ejection fraction (LVEF) <35%, LVEF 35%-50% and LVEF >50%.Using a large PCI registry, data from patients with ACS between January 2005 and June 2017 who were alive at 30 days were analysed. Those patients taking BB at 30 days were compared with those who were not taking BB. The primary outcome was all-cause mortality. The mean follow-up was 5.3±3.5 years.Of the 17 562 patients, 83.3% were on BB. Mortality was lower in the BB group (13.1% vs 19.5%, p=0.0001). Multivariable Cox proportional hazards model showed that BB use was associated with lower overall mortality (adjusted HR 0.87, 95% CI 0.78 to 0.97, p=0.014). In the subgroup analysis, BB use was associated with reduced mortality in LVEF <35% (adjusted HR 0.63, 95% CI 0.44 to 0.91, p=0.013), LVEF 35%-50% (adjusted HR 0.80, 95% CI 0.68 to 0.95, p=0.01), but not LVEF >50% (adjusted HR 1.03, 95% CI 0.87 to 1.21, p=0.74).BB use remains high and is associated with reduced mortality. This reduction in mortality is primarily seen in those with reduced ejection fraction, but not in those with preserved ejection fraction.