Abstract
Introduction: To describe the Danish National Health Service Register in relation to research. Content: The register contains data collected for administrative and scientific purposes from health contractors in primary health care. It includes information about citizens, providers, and health services but minimal clinical information. Validity and coverage: The register covers everyone living in Denmark and data is available from 1990. No validity studies have been reported. Because the data is connected to reimbursement the coverage is assumed to be good. CONCLUSION: The strengths of the register include completeness, size, and long follow-up period. It is useful for research purposes but reservations must be made regarding its validity.