A critical and unresolved issue in patient safety is how to determine whether unexpected harm, including death, could have been prevented. This retrospective study used medical record review to uncover if medical error occurred in cases of death in the hospital within 72 hours of an emergency department visit. If errors were present, clinicians determined whether errors could have been averted. Investigators found failure to initiate the correct treatment and failure to order a needed diagnostic test were both common errors, consistent with prior studies. More than half of unexpected deaths in the sample were related to a preventable medical error, suggesting that interventions to identify and ameliorate errors in the emergency department are urgently needed. A past AHRQ WebM&M perspective describes challenges of the emergency department setting in particular.