This study addressing the causes of missed and delayed diagnoses in emergency department patients used similar methodology as a companion study of error in the ambulatory setting and a prior study of surgical patients. Errors involved a broad range of patients and conditions. As in the outpatient arena, errors generally occurred due to failure to order diagnostic tests or interpret them correctly; factors contributing to error included cognitive factors (ie, physician judgment or knowledge), but system factors (ie, fatigue or communication breakdowns) were involved in a significant proportion of cases. As was also found in the study of ambulatory patients, the multifactorial nature of the errors identifies many potential areas for action but likely defies simple solutions.