The reliability of the Human Factors Analysis and Classification System (HFACS) for classifying retrospective observational human factors data in the cardiovascular operating room is examined. Three trained analysts independently used HFACS to categorize observational human factors data collected at a teaching and nonteaching hospital system. Results revealed that the framework was substantially reliable overall (Study I: k = 0.635; Study II: k = 0.642). Reliability increased when only preconditions for unsafe acts were investigated (Study I: k =0.660; Study II: k = 0.726). Preconditions for unsafe acts were the most commonly identified issues, with HFACS categories being similarly populated across both hospitals. HFACS is a reliable tool for systematically categorizing observational data of human factors issues in the operating room. Findings have implications for the development of a HFACS tool for proactively collecting observational human factors data, eliminating the necessity for classification post hoc.