The extent of unnecessary surgery has been the object of considerable speculation and occasional wild accusation in recent years. Most evidence of the existence of unnecessary surgery, such as information from studies of geographic variations and the results of second surgical opinion programs, is circumstantial. However, results from the few studies that have measured unnecessary surgery directly indicate that for some highly controversial operations the fraction that are unwarranted could be as high as 30 percent. Most unnecessary surgery results from physician uncertainty about the effectiveness of an operation. Elimination of this uncertainty requires more efficient production and dissemination of scientific information about clinical effectiveness. In the absence of adequate data from scientific studies, the use of a consensus of expert opinion, disseminated by means of comprehensive practice guidelines, offers the best opportunity to identify and eliminate unnecessary surgery.