Medical practice guidelines are being embraced enthusiastically by subspecialty and government organizations, but although hundreds have been developed at great public and private expense, there are few examples of how they have improved the quality of care or reduced health care costs. This article reports documented experience in changing physician behavior after implementing medical practice guidelines in the coronary care unit without altering physician incentives or imposing the threat of sanctions. Guideline adoption did require significant attention to the implementation process, including endorsement of guidelines by local "opinion leaders," validation by local data, and an intensive and concurrent implementation strategy.