The urgent need for information about clinical drug effects may be partially met by a large-scale comprehensive drugsurveillance program designed to permit the detection of unsuspected side effects and drug interactions, the quantitation of known effects, and the evaluation of the role of influencing factors. Such a program, the Boston Collaborative Drug Surveillance Program, now involving eight hospitals, is described, the principles and potential of this type of research are discussed, and data on some broad aspects of drug utilization and effects are presented. An appropriately designed drug-surveillance program can be introduced to hospital wards with a minimum of interference with the ward routine, can be applied in a standardized form in different centers, and is indeed capable of providing valuable information about clinical drug effects.