Background: Appropriate management of adverse medical events (AMEs) is the corner-stone of patient safety. Root cause analysis (RCA) is used to investigate serious or high-frequency errors, but is limited by its inability to explain a broader scope of factors. We re-reviewed RCA cases to understand the underlying human, organizational, or systemic factors affecting AMEs. Methods: A total of 40 consecutive RCA cases (2012-2016) were retrieved from the AMEs database. A panel was organized to retrospectively re-review these cases using the Human Factor Analysis and Classification System HFACS. Results: For active failures, errors stemmed largely from performance-based (95%), judgment and deci-sion-making errors (87.5%). Incorrectly followed procedures (81.6%) and accidental equipment operation (50%) were the most common types of performance-based errors. In-adequate real-time assessment (68.6%) and inappropriate operative actions (68.6%) were the most common decision-making errors. For sources of latent failure, teamwork problems (27.5%), including failure to effectively communicate (81.8%), and communicate critical information (72.7%) were common. Inadequate supervision (92.9%) or command oversights (92.9%) were the most common problems related to inadequate supervision (35%). Organizational program/policy risks not adequately assessed (50%) were the most common problems related to policy and process problem (25%). Conclusions: The HFACS review enhances our understanding of human factors underlying AMEs. The HFACS reveals latent supervisory, organizational, or systematic problems that cannot be addressed through tradi-tional RCA.