The hospital industry has recently experienced substantial merger activity. This paper examines several actual and proposed hospital mergers to determine the extent of competition in the affected markets and the effect these mergers may have on competition. Our focus is on mergers between hospitals in the same market. We define the relevant product and geographic market for hospitals, then develop criteria for evaluating hospital mergers and analyze several merger cases using these criteria. We conclude that these mergers threaten the competition that exists in most of the markets discussed, and that the claimed efficiency justification for mergers is not convincing.