BACKGROUND: Understanding bystander reactions to an emergency is an important component of effective training. Four stages of bystander intervention (BI) have been previously described: noticing the situation as a problem, interpreting when it is appropriate to intervene, recognizing personal responsibility to intervene, and knowing how to intervene. Using virtual reality (VR) to simulate emergencies such as sudden cardiac arrest (SCA) can be used to study these stages. METHODS: In a secondary analysis of an observational cohort study, we analyzed bystander self-efficacy for stages of BI before and after simulated SCA. Each subject participated in a singleplayer, immersive, VR SCA scenario. Subjects interacted with simulated bystanders through voice commands (“call 911”, “get an AED”). Actions taken in scenario, like performing CPR, were documented. Scenario BI actions were compared based on dichotomized comfort/discomfort. RESULTS: From June 2016 to June 2017, 119 subjects participated. Average age was 37±14 years, 44% were female and 46% reported CPR training within 2 years. During the scenario, 98% “noticed the event” and “interpreted it as a problem”, 78% “took responsibility”, and 54% “possessed the necessary skills”. Self-efficacy increased from pre- to post-scenario: noticing the event increased from 80% to 96%; interpreting as a problem increased from 86% to 97%; taking responsibility increased from 56% to 93%; possessing necessary skills increased from 47% to 63% (P<0.001). CONCLUSION Self-efficacy to respond to an SCA event increased pre- to post-scenario. Bystanders who reported feeling comfortable “taking responsibility to intervene” during an emergency were more likely to take action during a simulated emergency.