Pain is a significant health problem, and there is considerable need for clinical and epidemiological research in this topic. A prerequisite for doing research on patients treated with strong analgesics is that it is possible to identify the patients. We assessed two Danish population-based information systems, in which patients treated with strong analgesics are registered by using the patients' personal registration numbers as identifier. The two systems, which we compared, were (1) a surveillance system administered by the National Board of Health, and (2) the drug prescription register in the Danish National Health Service. During August 1994, 3787 patients were registered in the surveillance system and 3812 in the National Health Service in North Jutland County. Ninety-five persons were registered only in the surveillance system, and 120 only in the National Health Service register. A capture-recapture analysis showed a coverage of 96.9% for the surveillance system and 97.5% for the National Health Service. We thus conclude that the two systems form a valuable study base of patients treated with strong analgesics in epidemiological research.