Health maintenance organizations are covering 25% of U.S. residents this year, up from 23% in 2005.

Researchers at HealthLeaders-InterStudy, a unit of Decision Resources Inc., Waltham, Mass., have published those figures in a summary of research based on the company’s county-level managed care market report.

The number of U.S. residents enrolled in HMOs increased to 74 million this year, from about 68 million in 2005, according to HealthLeaders-InterStudy.

But researchers note that the increase in HMO market share may be somewhat misleading, because HMOs have been operating more like preferred provider organization plans, and PPO plans have been operating more like HMOs.