Virtually all health insurance markets are "highly concentrated," meaning insurer consolidation may have harmful effects on patients, physicians, employers and the economy, the American Medical Association recently reported. The report compares enrollment in commercial health plans 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. The association urged the Justice Department to study health insurance 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 consumers and providers.