A Consumer Guide to Coronary Artery Bypass Graft Surgery, published annually since 1992 by the Pennsylvania Health Care Cost Containment Council, compares the outcomes and charges for the state's hospitals and surgeons providing this surgery. To determine whether performance data caused hospitals to change their policies and practices, hospitals were surveyed in Pennsylvania, where the state releases annual coronary artery bypass graft (CABG) outcomes data and, as a control, in New Jersey, where the state does not release these data. Key informants representing hospitals, health insurance payers, health maintenance organizations, and purchasers were asked to list specific changes made because of comparative performance data released in public reports. Focus groups were conducted and surveys were then developed and administered to samples of hospitals, payers, and purchasers in both states. The results suggested, for example, that access to performance information encouraged hospitals to implement new approaches to marketing their CABG services. Thirty-eight percent of Pennsylvania CABG hospitals reported using performance information to recruit staff thoracic surgeons and residents, compared with none in New Jersey. For the most frequently initiated changes in patient care, the Pennsylvania hospitals depended on performance information released by a "government agency" to a much greater degree than did the hospitals in New Jersey. The results suggest that public release of performance information has encouraged hospitals in Pennsylvania to make changes in the areas of marketing, governance, and clinical care and that the impact of the release of public data on performance was greater in Pennsylvania hospitals than New Jersey hospitals.