Introduction: The Danish National Patient Register (NPR) was established in 1977, and it is considered to be the finest of its kind internationally.
Content: At the onset the register included information on inpatient in somatic wards. The content of the register has gradually been expanded, and since 2007 the register has included information on all patients in Danish hospitals.
Validity and coverage: Although the NPR is overall a sound data source, both the content and the definitions of single variables have changed over time. Changes in the organisation and provision of health services may affect both the type and the completeness of registrations.
Conclusion: The NPR is a unique data source. Researchers using the data should carefully consider potential fallacies in the data before drawing conclusions.