OBJECTIVE: To assess the utility of data already existing within hospitals for monitoring patient safety. SETTING: An acute hospital in southern England. DESIGN: Mapping of data sources proposed by staff as potentially able to identify patient safety issues followed by an in-depth analysis of the content of seven key sources. Data source analysis: For each data source: scope and depth of content in relation to patient safety, number and type of patient safety incidents identified, degree of overlap with incidents identified by different sources, levels of patient harm associated with incidents. RESULTS: A wide range of data sources existing within the hospital setting have the potential to provide information about patient safety incidents. Poor quality of coding, delays in reports reaching databases, the narrow focus of some data sources, limited data-collection periods and lack of central collation of findings were some of the barriers to making the best use of routine data sources for monitoring patient safety. An in-depth analysis of seven key data sources (Clinical Incident database, Health and Safety Incident database, Complaints database, Claims database and Inquest database, the Patient Administration System and case notes) indicated that case notes have the potential to identify the largest number of incidents and provide the richest source of information on such incidents. The seven data sources identified different types of incidents with differing levels of patient harm. There was little overlap between the incidents identified by different sources. CONCLUSION: Despite issues related to the quality of coding, depth of information available and accessibility, triangulating information from more than one source can identify a broader range of incidents and provide additional information related to professional groups involved, types of patients affected and important contributory factors. Such an approach can provide a focus for further work and ultimately contributes to the identification of appropriate interventions that improve patient safety.