The share of workers in self-insured company health plans increased for the fourth straight year in 2011 to 58.5% of all private-sector employees, according to an employment research group.

The Employee Benefit Research Institute analyzed data collected by the Agency for Healthcare Research and Quality and found a 1 percentage point increase last year in the share of private-sector employees in health plans operated by their employers (PDF), instead of commercially available plans. Ten years earlier, 48.8% of private-sector employees were in such plans.

Much of the increase has been driven by larger firms. For instance, the share of workers in firms with at least 1,000 employees enrolled in self-insured plans rose from 55.4% in 1998 to 86.3% in 2011.

The increase of workers in self-insured plans comes amid continuing speculation by health policy experts over whether the Patient Protection and Affordable Care Act will drive more employers to self-insure to avoid the cost of expensive requirements on commercial plans, such as the minimum creditable coverage requirement.

“Employers generally, and small employers particularly, concerned about the rising cost of providing health coverage may view self-insurance as a better way to control expected cost increases,” Paul Fronstin, director of EBRI's Health Research and Education Program and author of the report, said in a news release. “This new analysis provides a baseline against which to measure future trends.”