General practice computers have been widely used in the United Kingdom for the last 10 years and there are over 30 different systems currently available. The commercially available databases are based on two of the most widely used systems--VAMP Medical and Meditel. These databases provide both longitudinal and cross-sectional data on between 1.8 and 4 million patients. Despite their availability only limited use has been made of them for epidemiological and health service research purposes. They are a unique source of population-based information and deserve to be better recognized. The advantages of general practice databases include the fact that they are population based with excellent prescribing data linked to diagnosis, age and gender. The problems are that their primary purpose is patient care and the database population is constantly changing, as well as the usual problems of bias and confounding that occur in any observational studies. The barriers to the use of general practice databases include the cost of access, the size of the databases and that they are not structured in a way that easily allows analysis. Proper utilization of these databases requires powerful computers, staff proficient in writing computer programs to facilitate analysis and epidemiologists skilled in their use. If these structural problems are overcome then the databases are an invaluable source of data for epidemiological studies.