National databases and rheumatology research I: longitudinal databases in Scandinavia.

Nationwide population-based longitudinal databases provide excellent resources for medical research in Scandinavia. These include the Population Registry, the Cancer Registry, the Cause of Death Registry, the Hospital Discharge Registry, and other registers, and are linkable to each other by the personal identification code. The registers have long historical backgrounds, and are regulated strictly by law. This article describes features of the national databases and provides some examples of rheumatology research that use these databases.