The Institute of Medicine report in 1999 spurred a national movement in patient safety and focused attention on medical error as a significant cause of preventable injury and death. Throughout the past decade, the medical community has gradually acknowledged the fallibility of medical science and imperfections of our health care organizations. Before significant progress can be made to improve safety in health care, we must better understand the sources of error. This article is presented as one step in the process of change. A framework for classifying factors that contributed to errors identified in the emergency department (ED) is presented. The framework is, in its most basic form, a comprehensive checklist of all the sources of error uncovered in the course of investigating hundreds of cases referred to Stroger Hospital's emergency medicine quality assurance committee throughout the past decade. It begins with a look at error in the ED and then looks beyond the ED to examine error in the context of the wider health care system. It incorporates ideas found in safety engineering, transportation safety, human factors engineering, and our own experience in an urban, public, teaching hospital ED.