The purpose of the article is to describe the Danish National Health Service Register and its value in primary health care research, using mainly general practice as an example. The Danish National Health Service Register is a data system available for counties and municipalities to manage the National Health Insurance covering primary health care providers. The counties use the register for administrative purposes, especially for the settling of accounts with providers. The register contains data on all citizens, providers, and health care services reimbursed by the health authorities, but holds no data regarding health status. The accuracy and degree of completeness of persons and variables in the National Health Service Register rests on the fact that most primary health care services in Denmark are reimbursed and, therefore, included in the data base. This tie to the economy of both health care authorities and providers is supposed to confer the final register with a high degree of completeness. It is a clear advantage that almost all citizens in Denmark are assigned to only one general practice, and that the attitude towards research is positive among Danish patients and doctors. The register has so far only been used occasionally for research purposes. To take advantage of the register for research purposes within clinical and health services research, however, one must possess not only a detailed knowledge of the Danish society, including the structure of the Danish health care system, but also an intimate acquaintance with rather complex agreement system and the actual interpretation of this.