Operating-room crises (e.g., cardiac arrest and massive hemorrhage) are common events in large hospitals but can be rare for individual clinicians. Successful management is difficult and complex. We sought to evaluate a tool to improve adherence to evidence-based best practices during such events. Operating-room teams from three institutions (one academic medical center and two community hospitals) participated in a series of surgical-crisis scenarios in a simulated operating room. Each team was randomly assigned to manage half the scenarios with a set of crisis checklists and the remaining scenarios from memory alone. The primary outcome measure was failure to adhere to critical processes of care. Participants were also surveyed regarding their perceptions of the usefulness and clinical relevance of the checklists. A total of 17 operating-room teams participated in 106 simulated surgical-crisis scenarios. Failure to adhere to lifesaving processes of care was less common during simulations when checklists were available (6% of steps missed when checklists were available vs. 23% when they were unavailable, P<0.001). The results were similar in a multivariate model that accounted for clustering within teams, with adjustment for institution, scenario, and learning and fatigue effects (adjusted relative risk, 0.28; 95% confidence interval, 0.18 to 0.42; P<0.001). Every team performed better when the crisis checklists were available than when they were not. A total of 97% of the participants reported that if one of these crises occurred while they were undergoing an operation, they would want the checklist used. In a high-fidelity simulation study, checklist use was associated with significant improvement in the management of operating-room crises. These findings suggest that checklists for use during operating-room crises have the potential to improve surgical care. (Funded by the Agency for Healthcare Research and Quality.).