The longstanding cornerstone of medicine "first, do no harm" exists because of the fragility of life and health during medical care encounters, and represents the medical profession's understanding that patient safety has always been an important part of quality health care. Recently, however, concerns and evidence have mounted about the complexities of the health care system potentially causing patient deaths and significant unintended adverse effects. With a major national interest in addressing patient safety issues, a wide spectrum of individuals and organizations are working toward developing methods and systems to detect, characterize, and report potentially preventable adverse events. These activities are crucial precursors to prioritizing areas for action and for studying the effects of approaches to reduce sources of medical error. As part of this activity, the Evidence-based Practice Center (EPC) at the University of California San Francisco and Stanford University (UCSF-Stanford), with collaboration from the University of California Davis, was commissioned by the Agency for Healthcare Research and Quality (AHRQ) to review and improve the evidence base related to potential patient safety indicators (PSIs) that can be developed from routinely collected administrative data. For the purposes of this report, PSIs refer to measures that screen for potential problems that patients experience resulting from exposure to the health care system, and that are likely amenable to prevention by changes at the level of the system. The primary goal of this report is to document the evidence from a variety of sources on potential measures of patient safety suitable for use based on hospital discharge abstract data. The approach to identification and evaluation of PSIs presented in this report serves as the basis for development of a third module for the AHRQ QI tool set. This third module will be the Patient Safety Indicators (PSIs), which focus on potentially preventable instances of harm to patients, such as surgical complications and other iatrogenic events. The two other modules are the Prevention Quality Indicators, based on hospital admissions that might have been avoided through high-quality outpatient care; and the Inpatient Quality Indicators, consisting of inpatient mortality, utilization of procedures for which there are questions of overuse, underuse, or misuse; as well as volume of procedures for which higher volume is consistently associated with lower mortality.