The health maintenance organization (HMO) industry has undergone a wave of national consolidations in recent years. The most notable among these were between United HealthCare and MetraHealth (1995), PacifiCare Health Systems and FHP International (1996), Aetna Life and Casualty and U.S. Healthcare (1996), and Aetna and Prudential's health care unit (1999). This paper examines HMO consolidation from 1994 to 1997, looking first at concentration at the national level and then at the consequences of national consolidations for local markets. Whereas earlier mergers may have caused only a small increase in the type of local market concentration that may increase prices, later and currently proposed mergers may be motivated by considerations of increasing local market concentration. However, the concentration-increasing effect of national mergers was offset by the concentration-decreasing effect of HMO entry and growth. The analyses suggest that antitrust policy still has a role to play in ensuring that HMO markets remain open to new entry and in evaluating the effect of national mergers on local market concentration.