We describe a powerful new information resource for pharmacovigilance, pharmaco-economic studies, clinical epidemiology and health service research, which involves automated linkage of demographic, diagnostic and prescription information on some 3.4 million patients enrolled in over 500 separate general practices. Information is subject to regular validation checks. Access to this resource is available for bona fide research workers, subject to appropriate safeguards for patient anonymity and for scientific and ethical standards of the proposed studies. This database has depended upon the participating general practitioners and has already resulted in a substantial output of studies in peer-reviewed literature. It has great potential for further work of major relevance to public health, and its increasing use is to be actively encouraged.