The 800-pound gorilla, the Blue Cross and Blue Shield plans that have dominated many health insurance markets, may be losing strength.

The 15 largest Blue Cross- and Blue Shield-branded managed care organizations together lost more than 1 million members in the first six months of last year, according to a HealthLeaders-InterStudy analysis.

“Blue Cross and Blue Shield-branded plans typically dominate the markets in which they operate, and have traditionally shown the most solid enrollment growth compared to non-Blue plans,” Jane DuBose, director of health plan analysis for HealthLeaders-InterStudy, said in a statement. “However, we’re seeing that Blue plans have been losing members to aggressive pricing from national insurers. In addition, national insurers have worked to expand their provider networks, making them more competitive.”

Among the 15, eight reported declines in their total enrollment, including Horizon Blue Cross Blue Shield of New Jersey and Blue Shield of California, according to HealthLeaders-InterStudy, a provider of managed care market intelligence, which analyzed data available in Managed Market Surveyor-Rx. The report did not indicate the level of losses for those companies, nor did it identify the other plans that lost members.

The trend against Blues plans could continue. “In light of the recession, we anticipate enrollment shifts will continue,” DuBose said.

Enrollment data in Managed Market Surveyor-Rx describes plan and funding type, including commercial lives (HMO, PPO, consumer-driven), Medicaid/Medicare and fully/self-insured.