The Danish National Health Register (DNHR) has existed since 1977. It receives information about all patients admitted to hospitals in Denmark, including administrative data as well as information about treatment/diagnosis. This makes the DNHR very interesting for research. The history of the DNHR, its structure and its way of functioning are described briefly. All existing surveys concerning the validity of the data in the DNHR are discussed and compared to that of other countries, primarily the Nordic countries. The conclusion is that the validity of the administrative data and the overall data concerning treatment is fine (66-99%). But these figures conceal large differences in specific diagnoses. Work with selected data requires the data to be confirmed in another registry or at the primary source. Future changes in the DNHR are summed up at the end.