The Danish Psychiatric Central Research Register (DPCRR) is nation-wide with registration of all psychiatric hospitalizations in Denmark for the 5.3 million inhabitants (Munk-Jorgensen and Mor-tensen, 1997). From January 1, 1995 the register included information on patients in psychiatric ambulatories and community psychiatry centers, also. General practitioners and private practicing psychiatrist do not report to the DPCRR. All inhabitants in Denmark have a unique person identification number (Civil Person Registration number, CPR-number) that can be logically checked for errors; so it can be established with great certainty if a patient has had contact to psychiatric service previously, irrespective of changes in name etc. No private psychiatric inpatient hospitals or department are in operation in Denmark, all are organized within public services and reporting to the DPCRR. The International Classification of Dis-eases, 10th Revision (World Health Organization, 1992) has been used in Denmark from January 1, 1994.
2. Method
2.1. The register