Information systems that store routinely recorded health data have evolved from being a byproduct of health care delivery into a powerful resource for clinical research. Advantages of large studies using routinely collected data include direct applicability of findings to routine clinical practice, potential to provide reassuring (“null”) findings or detect safety signals, and statistically stable estimates overall and from subgroup analyses.

Denmark has hundreds of linkable population-based registries and databases available for research. The diversity and high quality of these databases with essentially complete population coverage and follow-up, universal healthcare access, and deterministic linkage is a constellation that has earned Denmark the title of 'an epidemiologist’s dream'.

All registries are linkable using the civil registration numbers, part of the Danish Civil Registration System, which was established in 1968. The civil registration number is a unique 10-digit number assigned to all Danish residents at birth or upon immigration. Data on age and sex are available on 100% of the population, and each resident is followed from birth or immigration to death or emigration, with no loss to follow-up.

Data and biosamples have been collected for over half a century. The underlying data are highly structured and coded using standard international classifications.