Virtually all health insurance markets are "highly concentrated," meaning insurer consolidation may have harmful effects on patients, physicians, employers and the economy, according to the American Medical Association's latest report on competition in health insurance. The report compares enrollment in commercial health plans (health maintenance organizations and preferred provider organizations) in 43 states and 313 smaller geographical areas with a market concentration index used by federal regulations. In at least 24 states, the two largest health insurers had a combined market share of 70% or more in 2007, up from 18 states the previous year. To restore a competitive balance to health insurance markets, the association urged the U.S. Department of Justice to perform a retrospective study of health insurance mergers similar to that performed by the Federal Trade Commission on hospital mergers; commission new research to identify causes and consequences of health insurer market power; and create a system for predicting the effects health insurer mergers will have on consumer and provider markets. The AHA submitted similar recommendations to the DOJ in May.