Registry data: how to harvest the seed of others.

Registries hold data collected by others than the investigator. Detailed knowledge of coverage and coding procedures enables the epidemiologist to extract essential information from registries. Rules and regulations vary from country to country and can be restrictive (U.S.) or liberal (Sweden). Registries form sampling frames for case-control and other studies of rheumatic diseases. The epidemiological potential of registries is underutilized.