Background: A minority of acute coronary syndrome (ACS) cases are associated with ventricular arrhythmias (VA) and/or cardiac arrest (CA). We investigated the effect of VA/CA at time of ACS on long-term outcomes. Methods and Results: We analysed routine clinical data from 5 NHS Trusts in the United Kingdom, collected between 2010 and 2017, by the National Institute for Health Research Health Informatics Collaborative (NIHR HIC). 13,444 patients with ACS, of which 376 (2.8%) had concurrent VA, survived to hospital discharge and were followed up for a median of 3.42 years. Patients with VA or CA at index presentation had significantly increased risks of subsequent VA during follow-up (VA group: adjusted HR 4.15, 95% CI 2.42-7.09, CA group: adjusted HR 2.60 95% CI 1.23-5.48). Patients who suffered a CA in the context of ACS and survived to discharge also had a 36% increase in long-term mortality (adjusted hazard ratio 1.36 (95% 1.04-1.78)), though the concurrent diagnosis of VA alone during ACS did not affect all-cause mortality (adjusted HR 1.03, 95% CI 0.80-1.33). Conclusions: Patients who develop VA or CA during ACS, who survive to discharge, have increased risks of subsequent VA, while those who have CA during ACS also have an increase in long-term mortality. These individuals may represent a subgroup at greater risk of subsequent arrhythmic events due to intrinsically lower thresholds for developing VA.