Background: Resuscitation of patients with time-critical and life-threatening illness represents a cognitive challenge for emergency room (ER) clinicians. We designed a cognitive aid, the Emergency Protocols Handbook, to simplify clinical management and team processes. Resuscitation guidelines were reformatted into simple, single step-bystep pathways. This Australian randomised controlled trial tested the effectiveness of this cognitive aid in a simulated ER environment by observing team error rates when current resuscitation guidelines were followed, with and without the handbook. Methods: Resuscitation teams were randomised to manage two scenarios with the handbook and two without in a high-fidelity simulation centre. Each scenario was video-recorded. The primary outcome measure was error rates (the number of errors made out of 15 key tasks per scenario). Key tasks varied by scenario. Each team completed four scenarios and was measured on 60 key tasks. Participants were surveyed regarding their perception of the usefulness of the handbook. Results: Twenty-one groups performed 84 ER crisis simulations. The unadjusted error rate in the handbook group was 18.8% (121/645) versus 38.9% (239/615) in the non-handbook group. There was a statistically significant reduction of 54.0% (95% CI 49.9-57.9) in the estimated percentage error rate when the handbook was available across all scenarios 17.9% (95% CI 14.4-22.0%) versus 38.9% (95% CI 34.2-43.9%). Almost all (97%) participants said they would want to use this cognitive aid during a real medical crisis situation. Conclusion: This trial showed that by following the step-by-step, linear pathways in the handbook, clinicians more than halved their teams' rate of error, across four simulated medical crises. The handbook improves team performance and enables healthcare teams to reduce clinical error rates and thus reduce harm for patients.