General practitioners (GPs) possess a wealth of information on the health of their patients. Hence, they are in a unique position to gather information for research, education or management. The chief goal of the Registration Network Family Practices is to establish a computerized database containing certain patient characteristics and all relevant health problems excluding minor, temporary illnesses. The database can be seen as a dynamic population sampling frame of chronic and/or severe morbidity, also including risk factors and psychosocial problems. The best way to make use of the Registration Network Family Practices is by researchers identifying and sampling patients with particular health problems. The database contained patient characteristics and problemlists of 61,887 persons, on September 1, 1995. At that time 214,389 health problems had been entered in the database. The database is increasingly being used as a source of information for studies by researchers and students. Researchers find the database a useful tool, but they have to keep in mind that data on the process of care are not directly available. Furthermore, there is a limit to the number of studies which can be performed in the network practices, due to time limitations and the burden on the doctors and patients.