Reports on the comparative performance of physicians are becoming increasingly common. Little is known, however, about the credibility of these reports with target audiences or their influence on the delivery of medical services. Since 1992, Pennsylvania has published the Consumer Guide to Coronary Artery Bypass Graft Surgery, which lists annual risk-adjusted mortality rates for all hospitals and surgeons providing such surgery in the state. In 1995, we surveyed a randomly selected sample of 50 percent of Pennsylvania cardiologists and cardiac surgeons to find out whether they were aware of the guide and, if so, to determine their views on its usefulness, limitations, and influence on providers. Eighty-two percent of the cardiologists and all the cardiac surgeons were aware of the guide. Only 10 percent of these respondents reported that its mortality rates were "very important" in assessing the performance of a cardiothoracic surgeon. Less than 10 percent reported discussing the guide with more than 10 percent of their patients who were candidates for a coronary-artery bypass graft (CABG). Eighty-seven percent of the cardiologists reported that the guide had a minimal influence or none on their referral recommendations. For both groups, the most important limitations of the guide were the absence of indicators of quality other than mortality (cited by 78 percent), inadequate risk adjustment (79 percent), and the unreliability of data provided by hospitals and surgeons (53 percent). Fifty-nine percent of the cardiologists reported increased difficulty in finding surgeons willing to perform CABG surgery in severely ill patients who required it, and 63 percent of the cardiac surgeons reported that they were less willing to operate on such patients. The Consumer Guide to Coronary Artery Bypass Graft Surgery has limited credibility among cardiovascular specialists. It has little influence on referral recommendations and may introduce a barrier to care for severely ill patients. If publicly released performance reports are intended to guide the choice of providers without impeding access to medical care, a collaborative process involving physicians may enhance the credibility and usefulness of the reports.