RATIONALE, AIMS AND OBJECTIVES Management of post-operative pain is unsatisfactory worldwide. An estimated 240 million patients undergo surgery each year. Forty to 60% of these patients report clinically significant pain. Discrepancy exists between availability of evidence-based medicine (EBM)-derived knowledge about management of perioperative pain and increased implementation of related practices versus lack of improvement in patient-reported outcomes (PROs). We aimed to assist health care providers to optimize perioperative pain management by developing and validating a medical registry that measures variability in care, identifies best pain management practices and assists clinicians in decision making. METHODS PAIN OUT was established from 2009 to 2012 with funding from the European Commission. It now continues as a self-sustaining, not-for-profit project, targeting health care professionals caring for patients undergoing surgery. RESULTS The growing registry includes data from 40 898 patients, 60 hospitals and 17 countries. Collaborators upload data (demographics, clinical, PROs) from patients undergoing surgery in their hospital/ward into an Internet-based portal. Two modules make use of the data: (1) online, immediate feedback and benchmarking compares PROs across sites while offline analysis permits in-depth analysis; and (2) the case-based clinical decision support system offers practice-based treatment recommendations for individual patients; it is available now as a prototype. The Electronic Knowledge Library provides succinct summaries on perioperative pain management, supporting knowledge transfer and application of EBM. CONCLUSION PAIN OUT, a large, growing international registry, allows use of 'real-life' data related to management of perioperative pain. Ultimately, comparative analysis through audit, feedback and benchmarking will improve quality of care.