The Danish National Hospital Register (LPR) has collected nationwide data on all somatic hospital admissions since 1977, and since 1995 data on outpatients and emergency patients have been included as well. Numerous research projects have been undertaken in the national Danish context as well as in collaboration with international teams, and the LPR is truly a valuable source of data for health sciences, especially in epidemiology, health services research and clinical research. Nearly complete registration of somatic hospital events in Denmark is combined with ideal conditions for longterm follow-up due to the existence of a national system of unique person identification in a population of relative demographic stability. Examples of studies are provided for illustration within three main areas: I: Using LPR for surveillance of the occurrence of diseases and of surgical procedures, II: Using the Register as a sampling frame for longitudinal population based and clinical research, and III: Using the Register as a data source for monitoring outcomes. Data available from the Register as well as studies of the validity of the data are mentioned, and it is described how researchers may get access to the Register. The Danish National Hospital Register is well suited to contribute to international comparative studies with relevance for evidence-based medicine.