Postmarketing drug surveillance (PMS) assesses the epidemiology of drug use and monitors beneficial or harmful effects of drugs following marketing. If used systematically, PMS can substantially improve drug therapy in the United States. This can be accomplished by generating information on drug use and effects otherwise unavailable, by enabling a more efficient drug approval process, and by educating drug prescribers. However, to be successful, any PMS system will need a good deal of input from such prescribers.