Per Hviid Gundtoft,1–3 Claus Varnum,4 Alma Becic Pedersen,5 Søren Overgaard2,3 1Department of Orthopedics, Kolding Hospital, Kolding, 2Department of Orthopedic Surgery and Traumatology, Odense University Hospital, 3Institute of Clinical Research, University of Southern Denmark, Odense, 4Department of Orthopedics, Vejle Hospital, Vejle, 5Department of Clinical Epidemiology, Aarhus University Hospital, Aarhus, Denmark Aim of database: The aim of the Danish Hip Arthroplasty Register (DHR) is to continuously monitor and improve the quality of treatment of primary and revision total hip arthroplasty (THA) in Denmark. Study population: The DHR is a Danish nationwide arthroplasty register established in January 1995. All Danish orthopedic departments – both public and private – report to the register, and registration is compulsory. Main variables: The main variables in the register include civil registration number, indication for primary and revision surgery, operation date and side, and postoperative complications. Completeness of primary and revision surgery is evaluated annually and validation of a number of variables has been carried out. Descriptive data: A total of 139,525 primary THAs and 22,118 revisions have been registered in the DHR between January 1, 1995 and December 31, 2014. Since 1995, completeness of procedure registration has been high, being 97.8% and 92.0% in 2014 for primary THAs and revisions, respectively. Several risk factors, such as comorbidity, age, specific primary diagnosis and fixation types for failure of primary THAs, and postoperative complications, have been identified through the DHR. Approximately 9,000 primary THAs and 1,500 revisions are reported to the register annually. Conclusion: The DHR is important for monitoring and improvement of treatment with THA and is a valuable tool for research in THA surgery due to the high quality of prospective collected data with long-term follow-up and high completeness. The register can be used for population-based epidemiology studies of THA surgery and can be linked to a range of other national databases. Keywords: Danish Hip Arthroplasty Register, validation, completeness